diff --git a/cspell.yml b/cspell.yml
index 0ea1def96..1a7446cbe 100644
--- a/cspell.yml
+++ b/cspell.yml
@@ -21,6 +21,8 @@ words:
   - tatooine
   - zuck
   - zuckerberg
+  - brontie
+  - oneOf
 # Forbid Alternative spellings
 flagWords:
   - implementor
diff --git a/spec/Section 3 -- Type System.md b/spec/Section 3 -- Type System.md
index 8400233fa..35f7e9bd0 100644
--- a/spec/Section 3 -- Type System.md	
+++ b/spec/Section 3 -- Type System.md	
@@ -1568,7 +1568,7 @@ InputFieldsDefinition : { InputValueDefinition+ }
 Fields may accept arguments to configure their behavior. These inputs are often
 scalars or enums, but they sometimes need to represent more complex values.
 
-A GraphQL Input Object defines a set of input fields; the input fields are
+:: A GraphQL _Input Object_ defines a set of input fields; the input fields are
 scalars, enums, other input objects, or any wrapping type whose underlying base
 type is one of those three. This allows arguments to accept arbitrarily complex
 structs.
@@ -1723,6 +1723,9 @@ input ExampleInputObject {
       returns {true}.
    4. If input field type is Non-Null and a default value is not defined:
       1. The `@deprecated` directive must not be applied to this input field.
+   5. If the Input Object is a _OneOf Input Object_ then:
+      1. The type of the input field must be nullable.
+      2. The input field must not have a default value.
 3. If an Input Object references itself either directly or through referenced
    Input Objects, at least one of the fields in the chain of references must be
    either a nullable or a List type.
@@ -1765,6 +1768,72 @@ InputFieldDefaultValueHasCycle(field, defaultValue, visitedFields):
   - Return {InputObjectDefaultValueHasCycle(namedFieldType, fieldDefaultValue,
     nextVisitedFields)}.
 
+### OneOf Input Objects
+
+:: A _OneOf Input Object_ is a special variant of _Input Object_ where exactly
+one field must be set and non-null, all others being omitted. This is useful for
+representing situations where an input may be one of many different options.
+
+When using the type system definition language, the [`@oneOf`](#sec--oneOf)
+directive is used to indicate that an Input Object is a OneOf Input Object (and
+thus requires exactly one of its fields be provided):
+
+```graphql
+input UserUniqueCondition @oneOf {
+  id: ID
+  username: String
+  organizationAndEmail: OrganizationAndEmailInput
+}
+```
+
+In schema introspection, the `__Type.isOneOf` field will return {true} for OneOf
+Input Objects, and {false} for all other Input Objects.
+
+**Input Coercion**
+
+The value of a OneOf Input Object, as a variant of Input Object, must also be an
+input object literal or an unordered map supplied by a variable, otherwise a
+_request error_ must be raised.
+
+- Prior to construction of the coerced map via the input coercion rules of an
+  _Input Object_: the value to be coerced must contain exactly one entry and
+  that entry must not be {null} or the {null} literal, otherwise a _request
+  error_ must be raised.
+
+- All _Input Object_
+  [input coercion rules](http://localhost:3000/draft#sec-Input-Objects.Input-Coercion)
+  must also apply to an _OneOf Input Object_.
+
+- The resulting coerced map must contain exactly one entry and the value for
+  that entry must not be {null}, otherwise an _execution error_ must be raised.
+
+Following are additional examples of input coercion for a OneOf Input Object
+type with a `String` member field `a` and an `Int` member field `b`:
+
+```graphql example
+input ExampleOneOfInputObject @oneOf {
+  a: String
+  b: Int
+}
+```
+
+| Literal Value           | Variables                       | Coerced Value                                       |
+| ----------------------- | ------------------------------- | --------------------------------------------------- |
+| `{ a: "abc" }`          | `{}`                            | `{ a: "abc" }`                                      |
+| `{ b: 123 }`            | `{}`                            | `{ b: 123 }`                                        |
+| `$var`                  | `{ var: { a: "abc" } }`         | `{ a: "abc" }`                                      |
+| `{ a: null }`           | `{}`                            | Error: Value for member field {a} must be non-null  |
+| `$var`                  | `{ var: { a: null } }`          | Error: Value for member field {a} must be non-null  |
+| `{ a: $a }`             | `{}`                            | Error: Value for member field {a} must be specified |
+| `{ a: "abc", b: 123 }`  | `{}`                            | Error: Exactly one key must be specified            |
+| `{ a: 456, b: "xyz" }`  | `{}`                            | Error: Exactly one key must be specified            |
+| `$var`                  | `{ var: { a: "abc", b: 123 } }` | Error: Exactly one key must be specified            |
+| `{ a: "abc", b: null }` | `{}`                            | Error: Exactly one key must be specified            |
+| `{ a: "abc", b: $b }`   | `{}`                            | Error: Exactly one key must be specified            |
+| `{ a: $a, b: $b }`      | `{ a: "abc" }`                  | Error: Exactly one key must be specified            |
+| `{}`                    | `{}`                            | Error: Exactly one key must be specified            |
+| `$var`                  | `{ var: {} }`                   | Error: Exactly one key must be specified            |
+
 ### Input Object Extensions
 
 InputObjectTypeExtension :
@@ -1788,6 +1857,12 @@ defined.
    the previous Input Object.
 4. Any non-repeatable directives provided must not already apply to the previous
    Input Object type.
+5. The `@oneOf` directive must not be provided by an Input Object type
+   extension.
+6. If the original Input Object is a _OneOf Input Object_ then:
+   1. All fields of the Input Object type extension must be nullable.
+   2. All fields of the Input Object type extension must not have default
+      values.
 
 ## List
 
@@ -2013,6 +2088,9 @@ schema.
 GraphQL implementations that support the type system definition language should
 provide the `@specifiedBy` directive if representing custom scalar definitions.
 
+GraphQL implementations that support the type system definition language should
+provide the `@oneOf` directive if representing OneOf Input Objects.
+
 When representing a GraphQL schema using the type system definition language any
 _built-in directive_ may be omitted for brevity.
 
@@ -2227,3 +2305,20 @@ to the relevant IETF specification.
 ```graphql example
 scalar UUID @specifiedBy(url: "https://tools.ietf.org/html/rfc4122")
 ```
+
+### @oneOf
+
+```graphql
+directive @oneOf on INPUT_OBJECT
+```
+
+The `@oneOf` _built-in directive_ is used within the type system definition
+language to indicate an _Input Object_ is a _OneOf Input Object_.
+
+```graphql example
+input UserUniqueCondition @oneOf {
+  id: ID
+  username: String
+  organizationAndEmail: OrganizationAndEmailInput
+}
+```
diff --git a/spec/Section 4 -- Introspection.md b/spec/Section 4 -- Introspection.md
index d7f8e629f..7643017de 100644
--- a/spec/Section 4 -- Introspection.md	
+++ b/spec/Section 4 -- Introspection.md	
@@ -151,6 +151,8 @@ type __Type {
   inputFields(includeDeprecated: Boolean! = false): [__InputValue!]
   # must be non-null for NON_NULL and LIST, otherwise null.
   ofType: __Type
+  # must be non-null for INPUT_OBJECT, otherwise null.
+  isOneOf: Boolean
 }
 
 enum __TypeKind {
@@ -373,6 +375,8 @@ Fields\:
 - `inputFields` must return the set of input fields as a list of `__InputValue`.
   - Accepts the argument `includeDeprecated` which defaults to {false}. If
     {true}, deprecated input fields are also returned.
+- `isOneOf` must return {true} when representing a _OneOf Input Object_,
+  otherwise {false}.
 - All other fields must return {null}.
 
 **List**
diff --git a/spec/Section 5 -- Validation.md b/spec/Section 5 -- Validation.md
index 88f1e4048..488881f88 100644
--- a/spec/Section 5 -- Validation.md	
+++ b/spec/Section 5 -- Validation.md	
@@ -40,6 +40,11 @@ type Query {
   findDog(searchBy: FindDogInput): Dog
 }
 
+type Mutation {
+  addPet(pet: PetInput!): Pet
+  addPets(pets: [PetInput!]!): [Pet]
+}
+
 enum DogCommand {
   SIT
   DOWN
@@ -92,6 +97,23 @@ input FindDogInput {
   name: String
   owner: String
 }
+
+input CatInput {
+  name: String!
+  nickname: String
+  meowVolume: Int
+}
+
+input DogInput {
+  name: String!
+  nickname: String
+  barkVolume: Int
+}
+
+input PetInput @oneOf {
+  cat: CatInput
+  dog: DogInput
+}
 ```
 
 ## Documents
@@ -1462,6 +1484,12 @@ query goodComplexDefaultValue($search: FindDogInput = { name: "Fido" }) {
     name
   }
 }
+
+mutation addPet($pet: PetInput! = { cat: { name: "Brontie" } }) {
+  addPet(pet: $pet) {
+    name
+  }
+}
 ```
 
 Non-coercible values (such as a String into an Int) are invalid. The following
@@ -1477,6 +1505,24 @@ query badComplexValue {
     name
   }
 }
+
+mutation oneOfWithNoFields {
+  addPet(pet: {}) {
+    name
+  }
+}
+
+mutation oneOfWithTwoFields($dog: DogInput) {
+  addPet(pet: { cat: { name: "Brontie" }, dog: $dog }) {
+    name
+  }
+}
+
+mutation listOfOneOfWithNullableVariable($dog: DogInput) {
+  addPets(pets: [{ dog: $dog }]) {
+    name
+  }
+}
 ```
 
 ### Input Object Field Names
@@ -2003,8 +2049,8 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
 - Let {variableType} be the expected type of {variableDefinition}.
 - Let {locationType} be the expected type of the {Argument}, {ObjectField}, or
   {ListValue} entry where {variableUsage} is located.
-- If {locationType} is a non-null type AND {variableType} is NOT a non-null
-  type:
+- If {IsNonNullPosition(locationType, variableUsage)} AND {variableType} is NOT
+  a non-null type:
   - Let {hasNonNullVariableDefaultValue} be {true} if a default value exists for
     {variableDefinition} and is not the value {null}.
   - Let {hasLocationDefaultValue} be {true} if a default value exists for the
@@ -2015,6 +2061,15 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
   - Return {AreTypesCompatible(variableType, nullableLocationType)}.
 - Return {AreTypesCompatible(variableType, locationType)}.
 
+IsNonNullPosition(locationType, variableUsage):
+
+- If {locationType} is a non-null type, return {true}.
+- If the location of {variableUsage} is an {ObjectField}:
+  - Let {parentObjectValue} be the {ObjectValue} containing {ObjectField}.
+  - Let {parentLocationType} be the expected type of {ObjectValue}.
+  - If {parentLocationType} is a _OneOf Input Object_ type, return {true}.
+- Return {false}.
+
 AreTypesCompatible(variableType, locationType):
 
 - If {locationType} is a non-null type:
@@ -2103,6 +2158,30 @@ query listToNonNullList($booleanList: [Boolean]) {
 This would fail validation because a `[T]` cannot be passed to a `[T]!`.
 Similarly a `[T]` cannot be passed to a `[T!]`.
 
+Variables used for OneOf Input Object fields must be non-nullable.
+
+```graphql example
+mutation addCat($cat: CatInput!) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+
+mutation addCatWithDefault($cat: CatInput! = { name: "Brontie" }) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
+```graphql counter-example
+mutation addNullableCat($cat: CatInput) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
 **Allowing Optional Variables When Default Values Exist**
 
 A notable exception to typical variable type compatibility is allowing a