diff --git a/src/cmd/compile/internal/types2/api_test.go b/src/cmd/compile/internal/types2/api_test.go
index bf807c35be..c7a24fc3e5 100644
--- a/src/cmd/compile/internal/types2/api_test.go
+++ b/src/cmd/compile/internal/types2/api_test.go
@@ -13,6 +13,7 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 
 	. "cmd/compile/internal/types2"
@@ -2295,6 +2296,60 @@ func TestInstantiate(t *testing.T) {
 	}
 }
 
+func TestInstantiateConcurrent(t *testing.T) {
+	const src = `package p
+
+type I[P any] interface {
+	m(P)
+	n() P
+}
+
+type J = I[int]
+
+type Nested[P any] *interface{b(P)}
+
+type K = Nested[string]
+`
+	pkg := mustTypecheck(src, nil, nil)
+
+	insts := []*Interface{
+		pkg.Scope().Lookup("J").Type().Underlying().(*Interface),
+		pkg.Scope().Lookup("K").Type().Underlying().(*Pointer).Elem().(*Interface),
+	}
+
+	// Use the interface instances concurrently.
+	for _, inst := range insts {
+		var (
+			counts  [2]int      // method counts
+			methods [2][]string // method strings
+		)
+		var wg sync.WaitGroup
+		for i := 0; i < 2; i++ {
+			i := i
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+
+				counts[i] = inst.NumMethods()
+				for mi := 0; mi < counts[i]; mi++ {
+					methods[i] = append(methods[i], inst.Method(mi).String())
+				}
+			}()
+		}
+		wg.Wait()
+
+		if counts[0] != counts[1] {
+			t.Errorf("mismatching method counts for %s: %d vs %d", inst, counts[0], counts[1])
+			continue
+		}
+		for i := 0; i < counts[0]; i++ {
+			if m0, m1 := methods[0][i], methods[1][i]; m0 != m1 {
+				t.Errorf("mismatching methods for %s: %s vs %s", inst, m0, m1)
+			}
+		}
+	}
+}
+
 func TestInstantiateErrors(t *testing.T) {
 	tests := []struct {
 		src    string // by convention, T must be the type being instantiated
diff --git a/src/cmd/compile/internal/types2/interface.go b/src/cmd/compile/internal/types2/interface.go
index 872a3217c2..6623ff5575 100644
--- a/src/cmd/compile/internal/types2/interface.go
+++ b/src/cmd/compile/internal/types2/interface.go
@@ -112,6 +112,7 @@ func (t *Interface) String() string   { return TypeString(t, nil) }
 // Implementation
 
 func (t *Interface) cleanup() {
+	t.typeSet() // any interface that escapes type checking must be safe for concurrent use
 	t.check = nil
 	t.embedPos = nil
 }
diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
index 5408c7e77f..7c9a46f231 100644
--- a/src/cmd/compile/internal/types2/named.go
+++ b/src/cmd/compile/internal/types2/named.go
@@ -633,11 +633,18 @@ func (n *Named) expandUnderlying() Type {
 				old := iface
 				iface = check.newInterface()
 				iface.embeddeds = old.embeddeds
+				assert(old.complete) // otherwise we are copying incomplete data
 				iface.complete = old.complete
 				iface.implicit = old.implicit // should be false but be conservative
 				underlying = iface
 			}
 			iface.methods = methods
+			iface.tset = nil // recompute type set with new methods
+
+			// If check != nil, check.newInterface will have saved the interface for later completion.
+			if check == nil { // golang/go#61561: all newly created interfaces must be fully evaluated
+				iface.typeSet()
+			}
 		}
 	}
 
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 74d6294dff..aefa53603f 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -170,6 +170,7 @@ func (subst *subster) typ(typ Type) Type {
 			iface := subst.check.newInterface()
 			iface.embeddeds = embeddeds
 			iface.implicit = t.implicit
+			assert(t.complete) // otherwise we are copying incomplete data
 			iface.complete = t.complete
 			// If we've changed the interface type, we may need to replace its
 			// receiver if the receiver type is the original interface. Receivers of
@@ -185,6 +186,11 @@ func (subst *subster) typ(typ Type) Type {
 			// need to create new interface methods to hold the instantiated
 			// receiver. This is handled by Named.expandUnderlying.
 			iface.methods, _ = replaceRecvType(methods, t, iface)
+
+			// If check != nil, check.newInterface will have saved the interface for later completion.
+			if subst.check == nil { // golang/go#61561: all newly created interfaces must be completed
+				iface.typeSet()
+			}
 			return iface
 		}
 
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 363e6d48e9..cb1263863f 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -16,6 +16,7 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 
 	. "go/types"
@@ -2300,6 +2301,60 @@ func TestInstantiate(t *testing.T) {
 	}
 }
 
+func TestInstantiateConcurrent(t *testing.T) {
+	const src = `package p
+
+type I[P any] interface {
+	m(P)
+	n() P
+}
+
+type J = I[int]
+
+type Nested[P any] *interface{b(P)}
+
+type K = Nested[string]
+`
+	pkg := mustTypecheck(src, nil, nil)
+
+	insts := []*Interface{
+		pkg.Scope().Lookup("J").Type().Underlying().(*Interface),
+		pkg.Scope().Lookup("K").Type().Underlying().(*Pointer).Elem().(*Interface),
+	}
+
+	// Use the interface instances concurrently.
+	for _, inst := range insts {
+		var (
+			counts  [2]int      // method counts
+			methods [2][]string // method strings
+		)
+		var wg sync.WaitGroup
+		for i := 0; i < 2; i++ {
+			i := i
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+
+				counts[i] = inst.NumMethods()
+				for mi := 0; mi < counts[i]; mi++ {
+					methods[i] = append(methods[i], inst.Method(mi).String())
+				}
+			}()
+		}
+		wg.Wait()
+
+		if counts[0] != counts[1] {
+			t.Errorf("mismatching method counts for %s: %d vs %d", inst, counts[0], counts[1])
+			continue
+		}
+		for i := 0; i < counts[0]; i++ {
+			if m0, m1 := methods[0][i], methods[1][i]; m0 != m1 {
+				t.Errorf("mismatching methods for %s: %s vs %s", inst, m0, m1)
+			}
+		}
+	}
+}
+
 func TestInstantiateErrors(t *testing.T) {
 	tests := []struct {
 		src    string // by convention, T must be the type being instantiated
diff --git a/src/go/types/interface.go b/src/go/types/interface.go
index f2bb15e84b..5fe9b57c3f 100644
--- a/src/go/types/interface.go
+++ b/src/go/types/interface.go
@@ -151,6 +151,7 @@ func (t *Interface) String() string   { return TypeString(t, nil) }
 // Implementation
 
 func (t *Interface) cleanup() {
+	t.typeSet() // any interface that escapes type checking must be safe for concurrent use
 	t.check = nil
 	t.embedPos = nil
 }
diff --git a/src/go/types/named.go b/src/go/types/named.go
index 413eaada27..fae7341234 100644
--- a/src/go/types/named.go
+++ b/src/go/types/named.go
@@ -635,11 +635,18 @@ func (n *Named) expandUnderlying() Type {
 				old := iface
 				iface = check.newInterface()
 				iface.embeddeds = old.embeddeds
+				assert(old.complete) // otherwise we are copying incomplete data
 				iface.complete = old.complete
 				iface.implicit = old.implicit // should be false but be conservative
 				underlying = iface
 			}
 			iface.methods = methods
+			iface.tset = nil // recompute type set with new methods
+
+			// If check != nil, check.newInterface will have saved the interface for later completion.
+			if check == nil { // golang/go#61561: all newly created interfaces must be fully evaluated
+				iface.typeSet()
+			}
 		}
 	}
 
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 30c48e1bad..13d3dcbf1e 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -172,6 +172,7 @@ func (subst *subster) typ(typ Type) Type {
 			iface := subst.check.newInterface()
 			iface.embeddeds = embeddeds
 			iface.implicit = t.implicit
+			assert(t.complete) // otherwise we are copying incomplete data
 			iface.complete = t.complete
 			// If we've changed the interface type, we may need to replace its
 			// receiver if the receiver type is the original interface. Receivers of
@@ -187,6 +188,11 @@ func (subst *subster) typ(typ Type) Type {
 			// need to create new interface methods to hold the instantiated
 			// receiver. This is handled by Named.expandUnderlying.
 			iface.methods, _ = replaceRecvType(methods, t, iface)
+
+			// If check != nil, check.newInterface will have saved the interface for later completion.
+			if subst.check == nil { // golang/go#61561: all newly created interfaces must be completed
+				iface.typeSet()
+			}
 			return iface
 		}