diff --git a/README.md b/README.md
index 0947d3f..841faa2 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,7 @@ type DSU[T comparable] interface {
     Find(x T) T            // Find returns the representative element (root) of x
     Union(x, y T) bool     // Union merges the sets containing x and y
     Connected(x, y T) bool // Connected reports whether x and y are in the same set
+    IsMember(x T) bool     // IsMember reports whether x is present (no mutation)
     Groups() map[T][]T     // Groups returns all connected components
 }
 ```
diff --git a/compact/compact.go b/compact/compact.go
index 89ca6f0..cf6de16 100644
--- a/compact/compact.go
+++ b/compact/compact.go
@@ -100,6 +100,11 @@ func (dsu *DSU) Connected(x, y int) bool {
 	return dsu.Find(x) == dsu.Find(y)
 }
 
+// IsMember reports whether x is within the managed range [0, n).
+func (dsu *DSU) IsMember(x int) bool {
+	return dsu.boundsCheck(x)
+}
+
 // Groups returns a map from root -> slice of elements in that set.
 func (dsu *DSU) Groups() map[int][]int {
 	groups := make(map[int][]int)
diff --git a/compact/compact_test.go b/compact/compact_test.go
index 6a974b5..b85d260 100644
--- a/compact/compact_test.go
+++ b/compact/compact_test.go
@@ -40,6 +40,16 @@ func TestCompactGroups(t *testing.T) {
 	}
 }
 
+func TestCompactIsMember(t *testing.T) {
+	dsu := New(3)
+	if !dsu.IsMember(0) || !dsu.IsMember(2) {
+		t.Fatalf("expected in-range elements to be members")
+	}
+	if dsu.IsMember(-1) || dsu.IsMember(3) {
+		t.Fatalf("expected out-of-range elements to report not a member")
+	}
+}
+
 // TestNewNegativeSize ensures New() dos not panic when called with a negative size.
 func TestCompactNewNegativeSize(t *testing.T) {
 	_ = New(-5)
diff --git a/gdsu.go b/gdsu.go
index b9dcec4..8c587f4 100644
--- a/gdsu.go
+++ b/gdsu.go
@@ -12,6 +12,7 @@ package gdsu
 //   - Find:      return the canonical representative (root) of x's set.
 //   - Union:     merge sets containing x and y, return true if they were separate.
 //   - Connected: report whether x and y belong to the same set.
+//   - IsMember:  report whether x has been registered with the DSU.
 //   - Groups:    return all current sets as root -> slice of elements.
 type DSU[T comparable] interface {
 	// Find returns the representative (root) of the set containing x.
@@ -23,6 +24,9 @@ type DSU[T comparable] interface {
 	// Connected returns true iff x and y belong to the same set.
 	Connected(x, y T) bool
 
+	// IsMember reports whether x is present in the DSU without mutating the DSU.
+	IsMember(x T) bool
+
 	// Groups returns a mapping of each root to all elements in its set.
 	Groups() map[T][]T
 }
diff --git a/gdsu_test.go b/gdsu_test.go
index e83ad4f..dc104df 100644
--- a/gdsu_test.go
+++ b/gdsu_test.go
@@ -9,6 +9,7 @@ type fakeDSU[T comparable] struct{}
 func (f *fakeDSU[T]) Find(x T) T            { return x }
 func (f *fakeDSU[T]) Union(x, y T) bool     { return true }
 func (f *fakeDSU[T]) Connected(x, y T) bool { return true }
+func (f *fakeDSU[T]) IsMember(x T) bool     { return true }
 func (f *fakeDSU[T]) Groups() map[T][]T     { return map[T][]T{} }
 
 func TestInterfaceCompiles(t *testing.T) {
diff --git a/sparse/sparse.go b/sparse/sparse.go
index 0537577..c413eac 100644
--- a/sparse/sparse.go
+++ b/sparse/sparse.go
@@ -87,6 +87,12 @@ func (dsu *DSU[T]) Connected(x, y T) bool {
 	return dsu.Find(x) == dsu.Find(y)
 }
 
+// IsMember reports whether x has been seen by the DSU without adding it.
+func (dsu *DSU[T]) IsMember(x T) bool {
+	_, exists := dsu.parent[x]
+	return exists
+}
+
 // Groups returns a map from root -> slice of elements in that set.
 func (dsu *DSU[T]) Groups() map[T][]T {
 	groups := make(map[T][]T)
diff --git a/sparse/sparse_test.go b/sparse/sparse_test.go
index 5773773..ba3d704 100644
--- a/sparse/sparse_test.go
+++ b/sparse/sparse_test.go
@@ -66,6 +66,22 @@ func TestSparseFindCreatesNew(t *testing.T) {
 	}
 }
 
+func TestSparseIsMember(t *testing.T) {
+	dsu := New[int](1, 2)
+
+	if !dsu.IsMember(1) || !dsu.IsMember(2) {
+		t.Fatalf("expected initial elements to be members")
+	}
+	if dsu.IsMember(3) {
+		t.Fatalf("expected unseen element to report not a member")
+	}
+	// Calling IsMember on an unseen element must not mutate the DSU.
+	groups := dsu.Groups()
+	if len(groups) != 2 {
+		t.Fatalf("expected 2 singleton groups, got %d", len(groups))
+	}
+}
+
 // TestSparseUnionOnNewElements ensures Union() works even if elements were never added before.
 func TestSparseUnionOnNewElements(t *testing.T) {
 	dsu := New[int]()