diff --git a/packages/expo-updates-interface/CHANGELOG.md b/packages/expo-updates-interface/CHANGELOG.md
index a19af1d401d2fc..a7b9e8fdb1cd54 100644
--- a/packages/expo-updates-interface/CHANGELOG.md
+++ b/packages/expo-updates-interface/CHANGELOG.md
@@ -10,6 +10,10 @@
 
 ### 💡 Others
 
+### ⚠️ Notices
+
+- Documentation for new native interface. ([#43230](https://github.com/expo/expo/pull/43230) by [@douglowder](https://github.com/douglowder))
+
 ## 55.1.2 — 2026-02-16
 
 ### 🎉 New features
diff --git a/packages/expo-updates-interface/README.md b/packages/expo-updates-interface/README.md
index 87b6feef9e411a..714f1d86270330 100644
--- a/packages/expo-updates-interface/README.md
+++ b/packages/expo-updates-interface/README.md
@@ -1,13 +1,236 @@
 # expo-updates-interface
 
-Native interface for modules that optionally depend on expo-updates, e.g. expo-dev-launcher.
+Native interface for modules that optionally depend on expo-updates. This package provides a unified native API (iOS and Android) for querying the state of the updates system and subscribing to state machine transitions, without requiring a direct dependency on `expo-updates`.
+
+## Overview
+
+`expo-updates-interface` defines two levels of interface:
+
+- **`UpdatesInterface`** -- implemented by all updates controllers (enabled, disabled, and dev-launcher). Provides read-only properties describing the running update and a method to subscribe to state machine changes.
+- **`UpdatesDevLauncherInterface`** -- extends `UpdatesInterface` with additional methods used exclusively by `expo-dev-launcher` to fetch updates and manage the update lifecycle.
+
+A singleton **`UpdatesControllerRegistry`** provides access to the active controller that implements one or both of the above interfaces.
 
 ## API documentation
 
+### UpdatesControllerRegistry
+
+The registry provides the active updates controller as a weak reference, in the `controller` property. The reference will be null when `expo-updates` is not installed and compiled into the app. If `expo-updates` is present, the property is set automatically at startup.
+
+| Platform    | Access                                                |
+| ----------- | ----------------------------------------------------- |
+| **iOS**     | `UpdatesControllerRegistry.sharedInstance.controller` |
+| **Android** | `UpdatesControllerRegistry.controller?.get()`         |
+
+### UpdatesInterface
+
+All updates controllers implement this interface. It is available whether updates is enabled, disabled, or running under the dev client.
+
+#### Properties
+
+| Property                                  | Type               | Description                                                                                                   |
+| ----------------------------------------- | ------------------ | ------------------------------------------------------------------------------------------------------------- |
+| `isEnabled`                               | `Bool` / `Boolean` | Whether the updates system is enabled. Defaults to `false` when updates is disabled.                          |
+| `runtimeVersion`                          | `String?`          | The runtime version of the running app. Set when updates is enabled or the dev client is running.             |
+| `updateURL` (iOS) / `updateUrl` (Android) | `URL?` / `Uri?`    | The update URL configured for this app. Set when updates is enabled or the dev client is running.             |
+| `launchedUpdateId`                        | `UUID?`            | The ID of the currently running update. Only set when updates is enabled.                                     |
+| `embeddedUpdateId`                        | `UUID?`            | The ID of the update embedded in the app binary. Only set when updates is enabled.                            |
+| `launchAssetPath`                         | `String?`          | The local file path of the launch asset (JS bundle) for the running update. Only set when updates is enabled. |
+
+#### Methods
+
+##### `subscribeToUpdatesStateChanges`
+
+Registers a listener that will be called on updates state machine transitions. Returns a subscription object that can be used to unsubscribe.
+
+**iOS:**
+
+```swift
+func subscribeToUpdatesStateChanges(_ listener: any UpdatesStateChangeListener) -> UpdatesStateChangeSubscription
+```
+
+**Android (Kotlin):**
+
+```kotlin
+fun subscribeToUpdatesStateChanges(listener: UpdatesStateChangeListener): UpdatesStateChangeSubscription
+```
+
+### UpdatesStateChangeListener
+
+A listener protocol/interface that receives state machine transition events.
+
+**iOS:**
+
+```swift
+public protocol UpdatesStateChangeListener {
+  func updatesStateDidChange(_ event: [String: Any])
+}
+```
+
+**Android (Kotlin):**
+
+```kotlin
+interface UpdatesStateChangeListener {
+  fun updatesStateDidChange(event: Map<String, Any>)
+}
+```
+
+The `event` dictionary contains information about the state transition, matching the structure of the updates state machine events exposed by the `expo-updates` JS API.
+
+### UpdatesStateChangeSubscription
+
+Returned by `subscribeToUpdatesStateChanges`. Call `remove()` to unsubscribe and stop receiving state change events.
+
+**iOS:**
+
+```swift
+public protocol UpdatesStateChangeSubscription {
+  func remove()
+}
+```
+
+**Android (Kotlin):**
+
+```kotlin
+interface UpdatesStateChangeSubscription {
+  fun remove()
+}
+```
+
+### UpdatesDevLauncherInterface
+
+Extends `UpdatesInterface` with methods used by `expo-dev-launcher` to fetch and manage updates. This interface is only implemented by the dev-launcher updates controller.
+
+See the source files for the full method signatures:
+
+- **iOS:** [`UpdatesInterface.swift`](ios/EXUpdatesInterface/UpdatesInterface.swift)
+- **Android:** [`UpdatesInterface.kt`](android/src/main/java/expo/modules/updatesinterface/UpdatesInterface.kt)
+
+## Usage example
+
+### Reading update information (Kotlin)
+
+```kotlin
+import expo.modules.updatesinterface.UpdatesControllerRegistry
+
+val controller = UpdatesControllerRegistry.controller?.get()
+if (controller != null && controller.isEnabled) {
+  val updateId = controller.launchedUpdateId
+  val runtimeVersion = controller.runtimeVersion
+  // ...
+}
+```
+
+### Reading update information (Swift)
+
+```swift
+import EXUpdatesInterface
+
+if let controller = UpdatesControllerRegistry.sharedInstance.controller,
+   controller.isEnabled {
+  let updateId = controller.launchedUpdateId
+  let runtimeVersion = controller.runtimeVersion
+  // ...
+}
+```
+
+### Subscribing to state changes (Kotlin)
+
+```kotlin
+import expo.modules.updatesinterface.UpdatesControllerRegistry
+import expo.modules.updatesinterface.UpdatesStateChangeListener
+import expo.modules.updatesinterface.UpdatesStateChangeSubscription
+
+val controller = UpdatesControllerRegistry.controller?.get() ?: return
+
+val subscription = controller.subscribeToUpdatesStateChanges(object : UpdatesStateChangeListener {
+  override fun updatesStateDidChange(event: Map<String, Any>) {
+    // Handle state change event
+  }
+})
+
+// Later, to unsubscribe:
+subscription.remove()
+```
+
+### Subscribing to state changes (Swift)
+
+```swift
+import EXUpdatesInterface
+
+class MyListener: NSObject, UpdatesStateChangeListener {
+  func updatesStateDidChange(_ event: [String: Any]) {
+    // Handle state change event
+  }
+}
+
+let listener = MyListener()
+if let controller = UpdatesControllerRegistry.sharedInstance.controller {
+  let subscription = controller.subscribeToUpdatesStateChanges(listener)
+
+  // Later, to unsubscribe:
+  subscription.remove()
+}
+```
+
+## Installation in an Expo native module
+
+- The `expo-updates-interface` package should be added to the module's NPM dependencies. (The `expo-updates` package does not need to be added.)
+- The module's iOS podspec should have "EXUpdatesInterface" added to the pod dependencies, as in this example:
+
+```ruby
+Pod::Spec.new do |s|
+  s.name           = 'InterfaceDemo'
+  s.version        = '1.0.0'
+  s.platforms      = {
+    :ios => '15.1',
+    :tvos => '15.1'
+  }
+  s.static_framework = true
+
+  s.dependency 'ExpoModulesCore'
+  s.dependency 'EXUpdatesInterface'
+
+  # Swift/Objective-C compatibility
+  s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
+  }
+
+  s.source_files = "**/*.{h,m,mm,swift,hpp,cpp}"
+end
+```
+
+- The module's Android `build.gradle` should have this package added as a dependency, as in this example:
+
+```gradle
+android {
+  namespace "expo.modules.interfacedemo"
+  defaultConfig {
+    versionCode 1
+    versionName "0.7.6"
+  }
+  lintOptions {
+    abortOnError false
+  }
+}
+
+dependencies {
+  implementation project(':expo-updates-interface')
+}
+```
+
 ## Installation in managed Expo projects
 
+This package is included as a dependency of `expo-updates` and `expo-dev-client`. No separate installation is needed.
+
 ## Installation in bare React Native projects
 
+This package is included as a dependency of `expo-updates` and `expo-dev-client`. If you need to install it separately:
+
+```sh
+npx expo install expo-updates-interface
+```
+
 ## Contributing
 
 Contributions are very welcome! Please refer to guidelines described in the [contributing guide](https://github.com/expo/expo#contributing).