diff --git a/toolchain/check/check.cpp b/toolchain/check/check.cpp
index 4e2b31358072..2206f81655c4 100644
--- a/toolchain/check/check.cpp
+++ b/toolchain/check/check.cpp
@@ -25,6 +25,28 @@ namespace Carbon::Check {
   auto Handle##Name(Context& context, Parse::Name##Id parse_node) -> bool;
 #include "toolchain/parse/node_kind.def"
 
+// Handles the transformation of a SemIRLocation to a DiagnosticLocation.
+class SemIRLocationTranslator
+    : public DiagnosticLocationTranslator<SemIRLocation> {
+ public:
+  explicit SemIRLocationTranslator(
+      Parse::NodeLocationTranslator* node_translator, const SemIR::File* sem_ir)
+      : node_translator_(node_translator), sem_ir_(sem_ir) {}
+
+  auto GetLocation(SemIRLocation loc) -> DiagnosticLocation override {
+    if (loc.is_inst_id) {
+      return node_translator_->GetLocation(
+          sem_ir_->insts().GetParseNode(loc.inst_id));
+    } else {
+      return node_translator_->GetLocation(loc.node_location);
+    }
+  }
+
+ private:
+  Parse::NodeLocationTranslator* node_translator_;
+  const SemIR::File* sem_ir_;
+};
+
 struct UnitInfo {
   // A given import within the file, with its destination.
   struct Import {
@@ -57,7 +79,6 @@ struct UnitInfo {
   Unit* unit;
 
   // Emitter information.
-  // TODO: Augment the translator to translate InstIds for locations.
   Parse::NodeLocationTranslator translator;
   ErrorTrackingDiagnosticConsumer err_tracker;
   DiagnosticEmitter<Parse::NodeLocation> emitter;
@@ -179,8 +200,10 @@ static auto CheckParseTree(const SemIR::File& builtin_ir, UnitInfo& unit_info,
   // For ease-of-access.
   SemIR::File& sem_ir = **unit_info.unit->sem_ir;
 
-  Context context(*unit_info.unit->tokens, unit_info.emitter,
-                  *unit_info.unit->parse_tree, sem_ir, vlog_stream);
+  SemIRLocationTranslator translator(&unit_info.translator, &sem_ir);
+  Context::DiagnosticEmitter emitter(translator, unit_info.err_tracker);
+  Context context(*unit_info.unit->tokens, emitter, *unit_info.unit->parse_tree,
+                  sem_ir, vlog_stream);
   PrettyStackTraceFunction context_dumper(
       [&](llvm::raw_ostream& output) { context.PrintForStackDump(output); });
 
diff --git a/toolchain/check/context.cpp b/toolchain/check/context.cpp
index 19cbfa86233e..233007b83dc6 100644
--- a/toolchain/check/context.cpp
+++ b/toolchain/check/context.cpp
@@ -104,7 +104,7 @@ auto Context::DiagnoseDuplicateName(Parse::NodeId parse_node,
   CARBON_DIAGNOSTIC(NameDeclPrevious, Note,
                     "Name is previously declared here.");
   emitter_->Build(parse_node, NameDeclDuplicate)
-      .Note(insts().GetParseNode(prev_def_id), NameDeclPrevious)
+      .Note(prev_def_id, NameDeclPrevious)
       .Emit();
 }
 
@@ -123,11 +123,9 @@ auto Context::NoteIncompleteClass(SemIR::ClassId class_id,
   const auto& class_info = classes().Get(class_id);
   CARBON_CHECK(!class_info.is_defined()) << "Class is not incomplete";
   if (class_info.definition_id.is_valid()) {
-    builder.Note(insts().GetParseNode(class_info.definition_id),
-                 ClassIncompleteWithinDefinition);
+    builder.Note(class_info.definition_id, ClassIncompleteWithinDefinition);
   } else {
-    builder.Note(insts().GetParseNode(class_info.decl_id),
-                 ClassForwardDeclaredHere);
+    builder.Note(class_info.decl_id, ClassForwardDeclaredHere);
   }
 }
 
diff --git a/toolchain/check/context.h b/toolchain/check/context.h
index 7a28d4751295..c22a3f83f754 100644
--- a/toolchain/check/context.h
+++ b/toolchain/check/context.h
@@ -23,10 +23,31 @@
 
 namespace Carbon::Check {
 
+// Diagnostic locations produced by checking may be either a parse node
+// directly, or an inst ID which is later translated to a parse node.
+struct SemIRLocation {
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  SemIRLocation(SemIR::InstId inst_id) : inst_id(inst_id), is_inst_id(true) {}
+
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  SemIRLocation(Parse::NodeLocation node_location)
+      : node_location(node_location), is_inst_id(false) {}
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  SemIRLocation(Parse::NodeId node_id)
+      : SemIRLocation(Parse::NodeLocation(node_id)) {}
+
+  union {
+    SemIR::InstId inst_id;
+    Parse::NodeLocation node_location;
+  };
+
+  bool is_inst_id;
+};
+
 // Context and shared functionality for semantics handlers.
 class Context {
  public:
-  using DiagnosticEmitter = Carbon::DiagnosticEmitter<Parse::NodeLocation>;
+  using DiagnosticEmitter = Carbon::DiagnosticEmitter<SemIRLocation>;
   using DiagnosticBuilder = DiagnosticEmitter::DiagnosticBuilder;
 
   // A scope in which `break` and `continue` can be used.
diff --git a/toolchain/check/convert.cpp b/toolchain/check/convert.cpp
index 63595099f1f6..9e4237a83659 100644
--- a/toolchain/check/convert.cpp
+++ b/toolchain/check/convert.cpp
@@ -555,8 +555,7 @@ static auto ConvertStructToClass(Context& context, SemIR::StructType src_type,
                       "Cannot construct instance of abstract class. "
                       "Consider using `partial {0}` instead.",
                       std::string);
-    context.emitter().Emit(context.insts().GetParseNode(value_id),
-                           ConstructionOfAbstractClass,
+    context.emitter().Emit(value_id, ConstructionOfAbstractClass,
                            context.sem_ir().StringifyType(target.type_id));
     return SemIR::InstId::BuiltinError;
   }
@@ -864,8 +863,7 @@ static auto PerformCopy(Context& context, SemIR::InstId expr_id)
   // copyable, or how to perform the copy.
   CARBON_DIAGNOSTIC(CopyOfUncopyableType, Error,
                     "Cannot copy value of type `{0}`.", std::string);
-  context.emitter().Emit(context.insts().GetParseNode(expr_id),
-                         CopyOfUncopyableType,
+  context.emitter().Emit(expr_id, CopyOfUncopyableType,
                          context.sem_ir().StringifyType(type_id));
   return SemIR::InstId::BuiltinError;
 }
@@ -888,8 +886,7 @@ auto Convert(Context& context, Parse::NodeId parse_node, SemIR::InstId expr_id,
     // namespace names, and allow use of functions as values.
     CARBON_DIAGNOSTIC(UseOfNonExprAsValue, Error,
                       "Expression cannot be used as a value.");
-    context.emitter().Emit(sem_ir.insts().GetParseNode(expr_id),
-                           UseOfNonExprAsValue);
+    context.emitter().Emit(expr_id, UseOfNonExprAsValue);
     return SemIR::InstId::BuiltinError;
   }
 
@@ -1100,8 +1097,7 @@ static auto ConvertSelf(Context& context, Parse::NodeId call_parse_node,
             InCallToFunctionSelf, Note,
             "Initializing `{0}` parameter of method declared here.",
             llvm::StringLiteral);
-        builder.Note(context.insts().GetParseNode(self_param_id),
-                     InCallToFunctionSelf,
+        builder.Note(self_param_id, InCallToFunctionSelf,
                      addr_pattern ? llvm::StringLiteral("addr self")
                                   : llvm::StringLiteral("self"));
       });
diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index 2e70b141e606..7c53dea5aff1 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -73,8 +73,7 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId parse_node)
                           "Previously declared here.");
         context.emitter()
             .Build(parse_node, ClassRedeclarationDifferentIntroducer)
-            .Note(context.insts().GetParseNode(existing_id),
-                  ClassRedeclarationDifferentIntroducerPrevious)
+            .Note(existing_id, ClassRedeclarationDifferentIntroducerPrevious)
             .Emit();
       }
 
@@ -134,8 +133,7 @@ auto HandleClassDefinitionStart(Context& context,
     context.emitter()
         .Build(parse_node, ClassRedefinition,
                context.names().GetFormatted(class_info.name_id).str())
-        .Note(context.insts().GetParseNode(class_info.definition_id),
-              ClassPreviousDefinition)
+        .Note(class_info.definition_id, ClassPreviousDefinition)
         .Emit();
   } else {
     class_info.definition_id = class_decl_id;
@@ -280,7 +278,7 @@ auto HandleBaseDecl(Context& context, Parse::BaseDeclId parse_node) -> bool {
                       "Previous `base` declaration is here.");
     context.emitter()
         .Build(parse_node, BaseRepeated)
-        .Note(context.insts().GetParseNode(class_info.base_id), BasePrevious)
+        .Note(class_info.base_id, BasePrevious)
         .Emit();
     return true;
   }
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index 87565567e22b..60ab5e5828f3 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -224,8 +224,7 @@ auto HandleFunctionDefinitionStart(Context& context,
     context.emitter()
         .Build(parse_node, FunctionRedefinition,
                context.names().GetFormatted(function.name_id).str())
-        .Note(context.insts().GetParseNode(function.definition_id),
-              FunctionPreviousDefinition)
+        .Note(function.definition_id, FunctionPreviousDefinition)
         .Emit();
   } else {
     function.definition_id = decl_id;
diff --git a/toolchain/check/handle_interface.cpp b/toolchain/check/handle_interface.cpp
index 3a015ca70f6d..0ff351dc044f 100644
--- a/toolchain/check/handle_interface.cpp
+++ b/toolchain/check/handle_interface.cpp
@@ -109,8 +109,7 @@ auto HandleInterfaceDefinitionStart(
     context.emitter()
         .Build(parse_node, InterfaceRedefinition,
                context.names().GetFormatted(interface_info.name_id).str())
-        .Note(context.insts().GetParseNode(interface_info.definition_id),
-              InterfacePreviousDefinition)
+        .Note(interface_info.definition_id, InterfacePreviousDefinition)
         .Emit();
   } else {
     interface_info.definition_id = interface_decl_id;
diff --git a/toolchain/check/handle_name.cpp b/toolchain/check/handle_name.cpp
index 3f3f83e1842c..bc0173b6ee0f 100644
--- a/toolchain/check/handle_name.cpp
+++ b/toolchain/check/handle_name.cpp
@@ -26,8 +26,7 @@ static auto GetAsNameScope(Context& context, SemIR::InstId base_id)
                         "Member access into incomplete class `{0}`.",
                         std::string);
       auto builder =
-          context.emitter().Build(context.insts().GetParseNode(base_id),
-                                  QualifiedExprInIncompleteClassScope,
+          context.emitter().Build(base_id, QualifiedExprInIncompleteClassScope,
                                   context.sem_ir().StringifyTypeExpr(base_id));
       context.NoteIncompleteClass(base_as_class->class_id, builder);
       builder.Emit();
@@ -119,7 +118,7 @@ auto HandleMemberAccessExpr(Context& context,
                           "Member access into object of incomplete type `{0}`.",
                           std::string);
         return context.emitter().Build(
-            context.insts().GetParseNode(base_id), IncompleteTypeInMemberAccess,
+            base_id, IncompleteTypeInMemberAccess,
             context.sem_ir().StringifyType(base_type_id));
       })) {
     context.node_stack().Push(parse_node, SemIR::InstId::BuiltinError);
diff --git a/toolchain/check/handle_struct.cpp b/toolchain/check/handle_struct.cpp
index 89351c294a00..0b87700b9ab7 100644
--- a/toolchain/check/handle_struct.cpp
+++ b/toolchain/check/handle_struct.cpp
@@ -79,10 +79,9 @@ static auto DiagnoseDuplicateNames(Context& context,
       CARBON_DIAGNOSTIC(StructNamePrevious, Note,
                         "Field with the same name here.");
       context.emitter()
-          .Build(context.insts().GetParseNode(field_inst_id),
-                 StructNameDuplicate, construct.str(),
+          .Build(field_inst_id, StructNameDuplicate, construct.str(),
                  sem_ir.names().GetFormatted(field_inst.name_id).str())
-          .Note(context.insts().GetParseNode(it->second), StructNamePrevious)
+          .Note(it->second, StructNamePrevious)
           .Emit();
       return true;
     }
diff --git a/toolchain/check/return.cpp b/toolchain/check/return.cpp
index 123d2af328cf..2674e1b0c92c 100644
--- a/toolchain/check/return.cpp
+++ b/toolchain/check/return.cpp
@@ -29,32 +29,29 @@ static auto GetCurrentReturnedVar(Context& context) -> SemIR::InstId {
 }
 
 // Produces a note that the given function has no explicit return type.
-static auto NoteNoReturnTypeProvided(Context& context,
-                                     Context::DiagnosticBuilder& diag,
+static auto NoteNoReturnTypeProvided(Context::DiagnosticBuilder& diag,
                                      const SemIR::Function& function) {
   CARBON_DIAGNOSTIC(ReturnTypeOmittedNote, Note,
                     "There was no return type provided.");
-  diag.Note(context.insts().GetParseNode(function.decl_id),
-            ReturnTypeOmittedNote);
+  diag.Note(function.decl_id, ReturnTypeOmittedNote);
 }
 
 // Produces a note describing the return type of the given function.
 static auto NoteReturnType(Context& context, Context::DiagnosticBuilder& diag,
                            const SemIR::Function& function) {
-  // TODO: This is the location of the `fn` keyword. Find the location of the
-  // return type.
-  auto type_parse_node = context.insts().GetParseNode(function.decl_id);
   CARBON_DIAGNOSTIC(ReturnTypeHereNote, Note,
                     "Return type of function is `{0}`.", std::string);
-  diag.Note(type_parse_node, ReturnTypeHereNote,
+  // TODO: This is using the location of the `fn` keyword. Find the location of
+  // the return type.
+  diag.Note(function.decl_id, ReturnTypeHereNote,
             context.sem_ir().StringifyType(function.return_type_id));
 }
 
 // Produces a note pointing at the currently in scope `returned var`.
-static auto NoteReturnedVar(Context& context, Context::DiagnosticBuilder& diag,
+static auto NoteReturnedVar(Context::DiagnosticBuilder& diag,
                             SemIR::InstId returned_var_id) {
   CARBON_DIAGNOSTIC(ReturnedVarHere, Note, "`returned var` was declared here.");
-  diag.Note(context.insts().GetParseNode(returned_var_id), ReturnedVarHere);
+  diag.Note(returned_var_id, ReturnedVarHere);
 }
 
 auto CheckReturnedVar(Context& context, Parse::NodeId returned_node,
@@ -68,7 +65,7 @@ auto CheckReturnedVar(Context& context, Parse::NodeId returned_node,
                       "Cannot declare a `returned var` in this function.");
     auto diag =
         context.emitter().Build(returned_node, ReturnedVarWithNoReturnType);
-    NoteNoReturnTypeProvided(context, diag, function);
+    NoteNoReturnTypeProvided(diag, function);
     diag.Emit();
     return SemIR::InstId::BuiltinError;
   }
@@ -101,9 +98,8 @@ auto RegisterReturnedVar(Context& context, SemIR::InstId bind_id) -> void {
     CARBON_DIAGNOSTIC(ReturnedVarShadowed, Error,
                       "Cannot declare a `returned var` in the scope of "
                       "another `returned var`.");
-    auto diag = context.emitter().Build(context.insts().GetParseNode(bind_id),
-                                        ReturnedVarShadowed);
-    NoteReturnedVar(context, diag, existing_id);
+    auto diag = context.emitter().Build(bind_id, ReturnedVarShadowed);
+    NoteReturnedVar(diag, existing_id);
     diag.Emit();
   }
 }
@@ -134,7 +130,7 @@ auto BuildReturnWithExpr(Context& context, Parse::ReturnStatementId parse_node,
         "No return expression should be provided in this context.");
     auto diag =
         context.emitter().Build(parse_node, ReturnStatementDisallowExpr);
-    NoteNoReturnTypeProvided(context, diag, function);
+    NoteNoReturnTypeProvided(diag, function);
     diag.Emit();
     expr_id = SemIR::InstId::BuiltinError;
   } else if (returned_var_id.is_valid()) {
@@ -142,7 +138,7 @@ auto BuildReturnWithExpr(Context& context, Parse::ReturnStatementId parse_node,
         ReturnExprWithReturnedVar, Error,
         "Can only `return var;` in the scope of a `returned var`.");
     auto diag = context.emitter().Build(parse_node, ReturnExprWithReturnedVar);
-    NoteReturnedVar(context, diag, returned_var_id);
+    NoteReturnedVar(diag, returned_var_id);
     diag.Emit();
     expr_id = SemIR::InstId::BuiltinError;
   } else if (function.return_slot_id.is_valid()) {