diff --git a/helper/schema/schema.go b/helper/schema/schema.go
index 2ac38803aa..3e5358b0d6 100644
--- a/helper/schema/schema.go
+++ b/helper/schema/schema.go
@@ -162,13 +162,30 @@ type Schema struct {
 	// used to wrap a complex structure, however less than one instance would
 	// cause instability.
 	//
-	// PromoteSingle, if true, will allow single elements to be standalone
-	// and promote them to a list. For example "foo" would be promoted to
-	// ["foo"] automatically. This is primarily for legacy reasons and the
-	// ambiguity is not recommended for new usage. Promotion is only allowed
-	// for primitive element types.
-	MaxItems      int
-	MinItems      int
+	// If the field Optional is set to true then MinItems is ignored and thus
+	// effectively zero.
+	//
+	// If MaxItems is 1, you may optionally also set AsSingle in order to have
+	// Terraform v0.12 or later treat a TypeList or TypeSet as if it were a
+	// single value. It will remain a list or set in Terraform v0.10 and v0.11.
+	// Enabling this for an existing attribute after you've made at least one
+	// v0.12-compatible provider release is a breaking change. AsSingle is
+	// likely to misbehave when used with deeply-nested set structures due to
+	// the imprecision of set diffs, so be sure to test it thoroughly,
+	// including updates that change the set members at all levels. AsSingle
+	// exists primarily to be used in conjunction with ConfigMode when forcing
+	// a nested resource to be treated as an attribute, so it can be considered
+	// an attribute of object type rather than of list/set of object.
+	MaxItems int
+	MinItems int
+	AsSingle bool
+
+	// PromoteSingle originally allowed for a single element to be assigned
+	// where a primitive list was expected, but this no longer works from
+	// Terraform v0.12 onwards (Terraform Core will require a list to be set
+	// regardless of what this is set to) and so only applies to Terraform v0.11
+	// and earlier, and so should be used only to retain this functionality
+	// for those still using v0.11 with a provider that formerly used this.
 	PromoteSingle bool
 
 	// The following fields are only valid for a TypeSet type.
@@ -775,6 +792,15 @@ func (m schemaMap) internalValidate(topSchemaMap schemaMap, attrsOnly bool) erro
 			}
 		}
 
+		if v.AsSingle {
+			if v.MaxItems != 1 {
+				return fmt.Errorf("%s: MaxItems must be 1 when AsSingle is set", k)
+			}
+			if v.Type != TypeList && v.Type != TypeSet {
+				return fmt.Errorf("%s: AsSingle can be used only with TypeList and TypeSet schemas", k)
+			}
+		}
+
 		// Computed-only field
 		if v.Computed && !v.Optional {
 			if v.ValidateFunc != nil {
diff --git a/helper/schema/schema_test.go b/helper/schema/schema_test.go
index 52d2a1961a..938681cb7a 100644
--- a/helper/schema/schema_test.go
+++ b/helper/schema/schema_test.go
@@ -3890,6 +3890,68 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			},
 			true, // in *schema.Resource with ConfigMode of attribute, so must also have ConfigMode of attribute
 		},
+
+		"AsSingle okay": {
+			map[string]*Schema{
+				"block": &Schema{
+					Type:     TypeList,
+					Optional: true,
+					MaxItems: 1,
+					AsSingle: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{
+							"sub": &Schema{
+								Type:     TypeList,
+								Optional: true,
+								Elem:     &Resource{},
+							},
+						},
+					},
+				},
+			},
+			false,
+		},
+
+		"AsSingle without MaxItems": {
+			map[string]*Schema{
+				"block": &Schema{
+					Type:     TypeList,
+					Optional: true,
+					AsSingle: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{
+							"sub": &Schema{
+								Type:     TypeList,
+								Optional: true,
+								Elem:     &Resource{},
+							},
+						},
+					},
+				},
+			},
+			true, // MaxItems must be 1 when AsSingle is set
+		},
+
+		"AsSingle on primitive type": {
+			map[string]*Schema{
+				"block": &Schema{
+					Type:     TypeString,
+					Optional: true,
+					MaxItems: 1,
+					AsSingle: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{
+							"sub": &Schema{
+								Type:     TypeList,
+								Optional: true,
+								Elem:     &Resource{},
+							},
+						},
+					},
+				},
+			},
+			true, // Unexpected error occurred: block: MaxItems and MinItems are only supported on lists or sets
+		},
 	}
 
 	for tn, tc := range cases {