diff --git a/runtime/vam/expr/function/flatten.go b/runtime/vam/expr/function/flatten.go
index a4889612a0..c76c42e5dd 100644
--- a/runtime/vam/expr/function/flatten.go
+++ b/runtime/vam/expr/function/flatten.go
@@ -31,3 +31,25 @@ func (f *flatten) Call(args ...vector.Any) vector.Any {
 	}
 	return builder.Build()
 }
+
+type unflatten struct {
+	fn *samfunc.Unflatten
+}
+
+func newUnflatten(zctx *super.Context) *unflatten {
+	return &unflatten{samfunc.NewUnflatten(zctx)}
+}
+
+func (u *unflatten) Call(args ...vector.Any) vector.Any {
+	vec := vector.Under(args[0])
+	typ := vec.Type()
+	builder := vector.NewDynamicBuilder()
+	var b zcode.Builder
+	for i := range vec.Len() {
+		b.Truncate()
+		vec.Serialize(&b, i)
+		val := u.fn.Call(nil, []super.Value{super.NewValue(typ, b.Bytes().Body())})
+		builder.Write(val)
+	}
+	return builder.Build()
+}
diff --git a/runtime/vam/expr/function/function.go b/runtime/vam/expr/function/function.go
index 84a25473d0..8bf03b4efd 100644
--- a/runtime/vam/expr/function/function.go
+++ b/runtime/vam/expr/function/function.go
@@ -113,10 +113,12 @@ func New(zctx *super.Context, name string, narg int) (expr.Function, field.Path,
 		f = &TypeName{zctx: zctx}
 	case "typeof":
 		f = &TypeOf{zctx}
-	case "upper":
-		f = &ToUpper{zctx}
 	case "under":
 		f = &Under{zctx}
+	case "unflatten":
+		f = newUnflatten(zctx)
+	case "upper":
+		f = &ToUpper{zctx}
 	default:
 		return nil, nil, function.ErrNoSuchFunction
 	}
diff --git a/runtime/sam/expr/function/ztests/unflatten.yaml b/runtime/ztests/expr/function/unflatten.yaml
similarity index 95%
rename from runtime/sam/expr/function/ztests/unflatten.yaml
rename to runtime/ztests/expr/function/unflatten.yaml
index 40ada5f181..71bbab1662 100644
--- a/runtime/sam/expr/function/ztests/unflatten.yaml
+++ b/runtime/ztests/expr/function/unflatten.yaml
@@ -1,4 +1,6 @@
-zed: "yield unflatten(this)"
+zed: yield unflatten(this)
+
+vector: true
 
 input: |
   [{key:["a","a"],value:1},{key:["a","b"],value:2},{key:["a","x","z"],value:"foo"},{key:["b"],value:2},{key:["c"],value:3}]