diff --git a/toolchain/check/testdata/facet/validate_impl_constraints.carbon b/toolchain/check/testdata/facet/validate_impl_constraints.carbon
index 7bb4063f244e..82e6a479a95f 100644
--- a/toolchain/check/testdata/facet/validate_impl_constraints.carbon
+++ b/toolchain/check/testdata/facet/validate_impl_constraints.carbon
@@ -452,3 +452,41 @@ fn F(T:! Z) {
   T.Z1 as Y;
   C(T.Z1) as Y;
 }
+
+// --- period_self_in_type_impls_named_constraint.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let Z1:! type;
+}
+interface Y {}
+
+constraint GivesY {
+  require impls Y;
+}
+
+class C(T:! type);
+
+fn F(T:! Z where C(.Self) impls GivesY) {
+  C(T) as Y;
+}
+
+// --- impl_witness_access_impls_named_constraint.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let Z1:! type;
+}
+interface Y {}
+
+constraint GivesY {
+  require impls Y;
+}
+
+// This type places a `.Self` (in the .Z1) into the identified facet type as
+// `.Self impls Y` through the named constraint. It ensures that the `.Self` is
+// correctly replaced by `T` in order to compare equal with `T.Z1` in impl
+// lookup.
+fn F(T:! Z where .Z1 impls GivesY) {
+  T.Z1 as Y;
+}
diff --git a/toolchain/check/type_completion.cpp b/toolchain/check/type_completion.cpp
index 18abb9a7e872..789e23b12298 100644
--- a/toolchain/check/type_completion.cpp
+++ b/toolchain/check/type_completion.cpp
@@ -983,14 +983,21 @@ static auto IdentifyFacetType(Context& context, SemIR::LocId loc_id,
   }
 
   struct SelfImplsFacetType {
+    // Whether the impling of facet type should be considered as extending in
+    // the resulting IdentifiedFacetType.
     bool extend;
+    // Whether we should replace `.Self` in the self type. This is false for the
+    // top-level self, since we'd be replacing parts of it with itself, which is
+    // cyclical. It becomes true when recursing into a `T impls ...` constraint
+    // where the self type is now something else.
+    bool subst_self;
     SemIR::ConstantId self;
     SemIR::FacetTypeId facet_type;
   };
 
   // Work queue.
   llvm::SmallVector<SelfImplsFacetType> work = {
-      {true, initial_self_const_id, facet_type.facet_type_id}};
+      {true, false, initial_self_const_id, facet_type.facet_type_id}};
 
   // Outputs for the IdentifiedFacetType.
   bool partially_identified = false;
@@ -1003,17 +1010,20 @@ static auto IdentifyFacetType(Context& context, SemIR::LocId loc_id,
   while (!work.empty()) {
     SelfImplsFacetType next_impls = work.pop_back_val();
     bool facet_type_extends = next_impls.extend;
+    auto subst_period_self_in_self = next_impls.subst_self;
     auto self_const_id = GetCanonicalFacetOrTypeValue(context, next_impls.self);
     const auto& facet_type_info =
         context.facet_types().Get(next_impls.facet_type);
 
-    auto self_and_interface = [&](SemIR::SpecificInterface interface)
+    auto self_and_interface = [&](SemIR::SpecificInterface impls_interface)
         -> SemIR::IdentifiedFacetType::RequiredImpl {
-      // Note that we subst `.Self` in the interface, but we do not in the
-      // self type here, as that would be cyclical, replacing part of the self
-      // type with itself.
-      return {self_const_id, SubstPeriodSelf(context, loc_id, interface,
-                                             period_self_replacement_id)};
+      auto self = subst_period_self_in_self
+                      ? SubstPeriodSelf(context, loc_id, self_const_id,
+                                        period_self_replacement_id)
+                      : self_const_id;
+      auto interface = SubstPeriodSelf(context, loc_id, impls_interface,
+                                       period_self_replacement_id);
+      return {self, interface};
     };
     auto type_and_interface =
         [&](SemIR::FacetTypeInfo::TypeImplsInterface impls)
@@ -1108,7 +1118,8 @@ static auto IdentifyFacetType(Context& context, SemIR::LocId loc_id,
                 .GetInstAs<SemIR::FacetType>(require_facet_type)
                 .facet_type_id;
         bool extend = facet_type_extends && require.extend_self;
-        work.push_back({extend, require_self, facet_type_id});
+        work.push_back(
+            {extend, subst_period_self_in_self, require_self, facet_type_id});
       }
     }
 
@@ -1164,7 +1175,8 @@ static auto IdentifyFacetType(Context& context, SemIR::LocId loc_id,
             context.constant_values()
                 .GetInstAs<SemIR::FacetType>(require_facet_type)
                 .facet_type_id;
-        work.push_back({false, require_self, facet_type_id});
+        work.push_back(
+            {false, subst_period_self_in_self, require_self, facet_type_id});
       }
     }
 
@@ -1225,7 +1237,7 @@ static auto IdentifyFacetType(Context& context, SemIR::LocId loc_id,
             context.constant_values()
                 .GetInstAs<SemIR::FacetType>(require_facet_type)
                 .facet_type_id;
-        work.push_back({false, require_self, facet_type_id});
+        work.push_back({false, true, require_self, facet_type_id});
       }
     }
   }