diff --git a/core/prelude/types/cpp/int.carbon b/core/prelude/types/cpp/int.carbon
index 23eb6e0266ef..1fdbd58021e0 100644
--- a/core/prelude/types/cpp/int.carbon
+++ b/core/prelude/types/cpp/int.carbon
@@ -250,3 +250,18 @@ impl CppCompat.Long32 as RightShiftAssignWith(CppCompat.Long32) {
 
 // TODO: ULong32, LongLong64, ULongLong64: compound assignments.
 // TODO: Long32: heterogeneous compound assignments.
+
+// Increment and decrement.
+impl CppCompat.Long32 as Dec {
+  fn Op[ref self: Self]() {
+    self -= (1 as CppCompat.Long32);
+  }
+}
+
+impl CppCompat.Long32 as Inc {
+  fn Op[ref self: Self]() {
+    self += (1 as CppCompat.Long32);
+  }
+}
+
+// TODO: Increment/decrement operations for ULong32, LongLong64, ULongLong64.
diff --git a/toolchain/check/testdata/interop/cpp/builtins.llp64.carbon b/toolchain/check/testdata/interop/cpp/builtins.llp64.carbon
index 8d7509abee9e..a942dd358a56 100644
--- a/toolchain/check/testdata/interop/cpp/builtins.llp64.carbon
+++ b/toolchain/check/testdata/interop/cpp/builtins.llp64.carbon
@@ -272,6 +272,20 @@ fn CompoundAssignmentHeterogeneousI64Long() {
   a += (1 as i64);
 }
 
+// --- increment_decrement_long.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn IncrementDecrementLong() {
+  //@dump-sem-ir-begin
+  var a: Cpp.long = 1;
+  ++a;
+  --a;
+  //@dump-sem-ir-end
+}
+
 // --- unsigned_long.carbon
 
 library "[[@TEST_NAME]]";
@@ -1858,6 +1872,86 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @DestroyOp(%self.param: %Cpp.long) = "no_op";
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- increment_decrement_long.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Cpp.long: type = class_type @Long32 [concrete]
+// CHECK:STDOUT:   %pattern_type.68c: type = pattern_type %Cpp.long [concrete]
+// CHECK:STDOUT:   %int_1.5b8: Core.IntLiteral = int_value 1 [concrete]
+// CHECK:STDOUT:   %ImplicitAs.type.819: type = facet_type <@ImplicitAs, @ImplicitAs(%Cpp.long)> [concrete]
+// CHECK:STDOUT:   %ImplicitAs.Convert.type.4c2: type = fn_type @ImplicitAs.Convert, @ImplicitAs(%Cpp.long) [concrete]
+// CHECK:STDOUT:   %ImplicitAs.impl_witness.2ce: <witness> = impl_witness imports.%ImplicitAs.impl_witness_table.903 [concrete]
+// CHECK:STDOUT:   %ImplicitAs.facet: %ImplicitAs.type.819 = facet_value Core.IntLiteral, (%ImplicitAs.impl_witness.2ce) [concrete]
+// CHECK:STDOUT:   %.dad: type = fn_type_with_self_type %ImplicitAs.Convert.type.4c2, %ImplicitAs.facet [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.type: type = fn_type @Core.IntLiteral.as.ImplicitAs.impl.Convert [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert: %Core.IntLiteral.as.ImplicitAs.impl.Convert.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.bound: <bound method> = bound_method %int_1.5b8, %Core.IntLiteral.as.ImplicitAs.impl.Convert [concrete]
+// CHECK:STDOUT:   %int_1.5a4: %Cpp.long = int_value 1 [concrete]
+// CHECK:STDOUT:   %Inc.type: type = facet_type <@Inc> [concrete]
+// CHECK:STDOUT:   %Inc.Op.type: type = fn_type @Inc.Op [concrete]
+// CHECK:STDOUT:   %Inc.impl_witness: <witness> = impl_witness imports.%Inc.impl_witness_table [concrete]
+// CHECK:STDOUT:   %Inc.facet: %Inc.type = facet_value %Cpp.long, (%Inc.impl_witness) [concrete]
+// CHECK:STDOUT:   %.153: type = fn_type_with_self_type %Inc.Op.type, %Inc.facet [concrete]
+// CHECK:STDOUT:   %Cpp.long.as.Inc.impl.Op.type: type = fn_type @Cpp.long.as.Inc.impl.Op [concrete]
+// CHECK:STDOUT:   %Cpp.long.as.Inc.impl.Op: %Cpp.long.as.Inc.impl.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Dec.type: type = facet_type <@Dec> [concrete]
+// CHECK:STDOUT:   %Dec.Op.type: type = fn_type @Dec.Op [concrete]
+// CHECK:STDOUT:   %Dec.impl_witness: <witness> = impl_witness imports.%Dec.impl_witness_table [concrete]
+// CHECK:STDOUT:   %Dec.facet: %Dec.type = facet_value %Cpp.long, (%Dec.impl_witness) [concrete]
+// CHECK:STDOUT:   %.fff: type = fn_type_with_self_type %Dec.Op.type, %Dec.facet [concrete]
+// CHECK:STDOUT:   %Cpp.long.as.Dec.impl.Op.type: type = fn_type @Cpp.long.as.Dec.impl.Op [concrete]
+// CHECK:STDOUT:   %Cpp.long.as.Dec.impl.Op: %Cpp.long.as.Dec.impl.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %DestroyOp.type: type = fn_type @DestroyOp [concrete]
+// CHECK:STDOUT:   %DestroyOp: %DestroyOp.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .long = constants.%Cpp.long
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import_ref.b8a: %Core.IntLiteral.as.ImplicitAs.impl.Convert.type = import_ref Core//prelude/types/cpp/int, loc{{\d+_\d+}}, loaded [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert]
+// CHECK:STDOUT:   %ImplicitAs.impl_witness_table.903 = impl_witness_table (%Core.import_ref.b8a), @Core.IntLiteral.as.ImplicitAs.impl.052 [concrete]
+// CHECK:STDOUT:   %Core.import_ref.9d7: %Cpp.long.as.Inc.impl.Op.type = import_ref Core//prelude/types/cpp/int, loc{{\d+_\d+}}, loaded [concrete = constants.%Cpp.long.as.Inc.impl.Op]
+// CHECK:STDOUT:   %Inc.impl_witness_table = impl_witness_table (%Core.import_ref.9d7), @Cpp.long.as.Inc.impl [concrete]
+// CHECK:STDOUT:   %Core.import_ref.d5d: %Cpp.long.as.Dec.impl.Op.type = import_ref Core//prelude/types/cpp/int, loc{{\d+_\d+}}, loaded [concrete = constants.%Cpp.long.as.Dec.impl.Op]
+// CHECK:STDOUT:   %Dec.impl_witness_table = impl_witness_table (%Core.import_ref.d5d), @Cpp.long.as.Dec.impl [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @IncrementDecrementLong() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %a.patt: %pattern_type.68c = ref_binding_pattern a [concrete]
+// CHECK:STDOUT:     %a.var_patt: %pattern_type.68c = var_pattern %a.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %a.var: ref %Cpp.long = var %a.var_patt
+// CHECK:STDOUT:   %int_1: Core.IntLiteral = int_value 1 [concrete = constants.%int_1.5b8]
+// CHECK:STDOUT:   %impl.elem0.loc8: %.dad = impl_witness_access constants.%ImplicitAs.impl_witness.2ce, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert]
+// CHECK:STDOUT:   %bound_method.loc8: <bound method> = bound_method %int_1, %impl.elem0.loc8 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call: init %Cpp.long = call %bound_method.loc8(%int_1) [concrete = constants.%int_1.5a4]
+// CHECK:STDOUT:   %.loc8_3: init %Cpp.long = converted %int_1, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call [concrete = constants.%int_1.5a4]
+// CHECK:STDOUT:   assign %a.var, %.loc8_3
+// CHECK:STDOUT:   %.loc8_13: type = splice_block %long.ref [concrete = constants.%Cpp.long] {
+// CHECK:STDOUT:     %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %long.ref: type = name_ref long, constants.%Cpp.long [concrete = constants.%Cpp.long]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %a: ref %Cpp.long = ref_binding a, %a.var
+// CHECK:STDOUT:   %a.ref.loc9: ref %Cpp.long = name_ref a, %a
+// CHECK:STDOUT:   %impl.elem0.loc9: %.153 = impl_witness_access constants.%Inc.impl_witness, element0 [concrete = constants.%Cpp.long.as.Inc.impl.Op]
+// CHECK:STDOUT:   %bound_method.loc9: <bound method> = bound_method %a.ref.loc9, %impl.elem0.loc9
+// CHECK:STDOUT:   %Cpp.long.as.Inc.impl.Op.call: init %empty_tuple.type = call %bound_method.loc9(%a.ref.loc9)
+// CHECK:STDOUT:   %a.ref.loc10: ref %Cpp.long = name_ref a, %a
+// CHECK:STDOUT:   %impl.elem0.loc10: %.fff = impl_witness_access constants.%Dec.impl_witness, element0 [concrete = constants.%Cpp.long.as.Dec.impl.Op]
+// CHECK:STDOUT:   %bound_method.loc10: <bound method> = bound_method %a.ref.loc10, %impl.elem0.loc10
+// CHECK:STDOUT:   %Cpp.long.as.Dec.impl.Op.call: init %empty_tuple.type = call %bound_method.loc10(%a.ref.loc10)
+// CHECK:STDOUT:   %DestroyOp.bound: <bound method> = bound_method %a.var, constants.%DestroyOp
+// CHECK:STDOUT:   %DestroyOp.call: init %empty_tuple.type = call %DestroyOp.bound(%a.var)
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @DestroyOp(%self.param: %Cpp.long) = "no_op";
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- unsigned_long.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
diff --git a/toolchain/check/testdata/interop/cpp/builtins.lp64.carbon b/toolchain/check/testdata/interop/cpp/builtins.lp64.carbon
index 242c4d3acb09..28e6bb57a237 100644
--- a/toolchain/check/testdata/interop/cpp/builtins.lp64.carbon
+++ b/toolchain/check/testdata/interop/cpp/builtins.lp64.carbon
@@ -559,8 +559,8 @@ fn F() {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Core.Int: %Int.type = import_ref Core//prelude/types/int, Int, loaded [concrete = constants.%Int.generic]
-// CHECK:STDOUT:   %Core.import_ref.91c: %Cpp.long_long.as.ImplicitAs.impl.Convert.type.194 = import_ref Core//prelude/types/cpp/int, loc{{\d+_\d+}}, loaded [concrete = constants.%Cpp.long_long.as.ImplicitAs.impl.Convert.c03]
-// CHECK:STDOUT:   %ImplicitAs.impl_witness_table.563 = impl_witness_table (%Core.import_ref.91c), @Cpp.long_long.as.ImplicitAs.impl.034 [concrete]
+// CHECK:STDOUT:   %Core.import_ref.91c3: %Cpp.long_long.as.ImplicitAs.impl.Convert.type.194 = import_ref Core//prelude/types/cpp/int, loc{{\d+_\d+}}, loaded [concrete = constants.%Cpp.long_long.as.ImplicitAs.impl.Convert.c03]
+// CHECK:STDOUT:   %ImplicitAs.impl_witness_table.563 = impl_witness_table (%Core.import_ref.91c3), @Cpp.long_long.as.ImplicitAs.impl.034 [concrete]
 // CHECK:STDOUT:   %LongResult.decl: type = class_decl @LongResult [concrete = constants.%LongResult] {} {}
 // CHECK:STDOUT:   %PassLongLong__carbon_thunk.decl.139b1c.2: %PassLongLong__carbon_thunk.type.aa8bde.2 = fn_decl @PassLongLong__carbon_thunk.2 [concrete = constants.%PassLongLong__carbon_thunk.9998b6.2] {
 // CHECK:STDOUT:     <elided>