diff --git a/crates/ruma-client-api/src/r0/keys/upload_keys.rs b/crates/ruma-client-api/src/r0/keys/upload_keys.rs
index e35e09d59b..45e97f602d 100644
--- a/crates/ruma-client-api/src/r0/keys/upload_keys.rs
+++ b/crates/ruma-client-api/src/r0/keys/upload_keys.rs
@@ -29,6 +29,11 @@ ruma_api! {
         /// One-time public keys for "pre-key" messages.
         #[serde(skip_serializing_if = "Option::is_none")]
         pub one_time_keys: Option<BTreeMap<Box<DeviceKeyId>, Raw<OneTimeKey>>>,
+
+        /// Fallback public keys for "pre-key" messages.
+        #[cfg(feature = "unstable-pre-spec")]
+        #[serde(skip_serializing_if = "Option::is_none", rename = "org.matrix.msc2732.fallback_keys")]
+        pub fallback_keys: Option<BTreeMap<Box<DeviceKeyId>, Raw<OneTimeKey>>>,
     }
 
     response: {
diff --git a/crates/ruma-client-api/src/r0/sync/sync_events.rs b/crates/ruma-client-api/src/r0/sync/sync_events.rs
index 37b58b1118..51fdb1122c 100644
--- a/crates/ruma-client-api/src/r0/sync/sync_events.rs
+++ b/crates/ruma-client-api/src/r0/sync/sync_events.rs
@@ -93,6 +93,15 @@ ruma_api! {
         /// currently held on the server for a device.
         #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
         pub device_one_time_keys_count: BTreeMap<DeviceKeyAlgorithm, UInt>,
+
+        /// For each key algorithm, the number of unclaimed one-time keys
+        /// currently held on the server for a device.
+        ///
+        /// The presence of this field indicates that the server supports
+        /// fallback keys.
+        #[cfg(feature = "unstable-pre-spec")]
+        #[serde(rename = "org.matrix.msc2732.device_unused_fallback_key_types")]
+        pub device_unused_fallback_key_types: Option<Vec<DeviceKeyAlgorithm>>,
     }
 
     error: crate::Error
@@ -116,6 +125,8 @@ impl Response {
             to_device: Default::default(),
             device_lists: Default::default(),
             device_one_time_keys_count: BTreeMap::new(),
+            #[cfg(feature = "unstable-pre-spec")]
+            device_unused_fallback_key_types: None,
         }
     }
 }
diff --git a/crates/ruma-common/src/encryption.rs b/crates/ruma-common/src/encryption.rs
index 14c5028fa1..1ce1ee2bea 100644
--- a/crates/ruma-common/src/encryption.rs
+++ b/crates/ruma-common/src/encryption.rs
@@ -83,12 +83,28 @@ pub struct SignedKey {
 
     /// Signatures for the key object.
     pub signatures: SignedKeySignatures,
+
+    /// Is this key considered to be a fallback key, defaults to false.
+    #[cfg(feature = "unstable-pre-spec")]
+    #[serde(default, skip_serializing_if = "ruma_serde::is_default")]
+    pub fallback: bool,
 }
 
 impl SignedKey {
     /// Creates a new `SignedKey` with the given key and signatures.
     pub fn new(key: String, signatures: SignedKeySignatures) -> Self {
-        Self { key, signatures }
+        Self {
+            key,
+            signatures,
+            #[cfg(feature = "unstable-pre-spec")]
+            fallback: false,
+        }
+    }
+
+    /// Creates a new fallback `SignedKey` with the given key and signatures.
+    #[cfg(feature = "unstable-pre-spec")]
+    pub fn new_fallback(key: String, signatures: SignedKeySignatures) -> Self {
+        Self { key, signatures, fallback: true }
     }
 }