diff --git a/src/r0/sync/sync_events.rs b/src/r0/sync/sync_events.rs
index efacb9b2..2055173b 100644
--- a/src/r0/sync/sync_events.rs
+++ b/src/r0/sync/sync_events.rs
@@ -68,12 +68,45 @@ pub enum SetPresence {
 #[derive(Clone, Debug, Deserialize, Serialize)]
 #[serde(untagged)]
 pub enum Filter {
+    // The filter definition needs to be (de)serialized twice because it is a URL-encoded JSON
+    // string. Since #[ruma_api(query)] only does the latter and this is a very uncommon
+    // setup, we implement it through custom serde logic for this specific enum variant rather than
+    // adding another ruma_api attribute.
+    //
+    // On the deserialization side, because this is an enum with #[serde(untagged)], serde will
+    // try the variants in order (https://serde.rs/enum-representations.html). That means because
+    // FilterDefinition is the first variant, JSON decoding is attempted first which is almost
+    // functionally equivalent to looking at whether the first symbol is a '{' as the spec says.
+    // (there are probably some corner cases like leading whitespace)
+    #[serde(with = "filter_def_serde")]
     /// A complete filter definition serialized to JSON.
     FilterDefinition(FilterDefinition),
     /// The ID of a filter saved on the server.
     FilterId(String),
 }
 
+mod filter_def_serde {
+    use serde::{de::Error as _, ser::Error as _, Deserialize, Deserializer, Serializer};
+
+    use crate::r0::filter::FilterDefinition;
+
+    pub fn serialize<S>(filter_def: &FilterDefinition, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let string = serde_json::to_string(filter_def).map_err(S::Error::custom)?;
+        serializer.serialize_str(&string)
+    }
+
+    pub fn deserialize<'de, D>(deserializer: D) -> Result<FilterDefinition, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let filter_str = <&str>::deserialize(deserializer)?;
+        serde_json::from_str(filter_str).map_err(D::Error::custom)
+    }
+}
+
 /// Updates to rooms.
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct Rooms {