diff --git a/crates/cli/src/commands/syn2mas.rs b/crates/cli/src/commands/syn2mas.rs
index dfa835b95..4ea55a21f 100644
--- a/crates/cli/src/commands/syn2mas.rs
+++ b/crates/cli/src/commands/syn2mas.rs
@@ -61,6 +61,14 @@ pub(super) struct Options {
     /// configure all values through those environment variables.
     #[clap(long = "synapse-database-uri", global = true)]
     synapse_database_uri: Option<PgConnectOptions>,
+
+    /// Make missing auth providers in Synapse config warnings instead of
+    /// errors. If this flag is set, and we find `auth_provider` values in
+    /// the Synapse `user_external_ids` table, that are not configured in
+    /// the Synapse OIDC configuration, instead of erroring we will just
+    /// output warnings.
+    #[clap(long = "ignore-missing-auth-providers", global = true)]
+    ignore_missing_auth_providers: bool,
 }
 
 #[derive(Parser, Debug)]
@@ -169,8 +177,13 @@ impl Options {
         // Check databases
         syn2mas::mas_pre_migration_checks(&mut mas_connection).await?;
         {
-            let (extra_warnings, extra_errors) =
-                syn2mas::synapse_database_check(&mut syn_conn, &synapse_config, figment).await?;
+            let (extra_warnings, extra_errors) = syn2mas::synapse_database_check(
+                &mut syn_conn,
+                &synapse_config,
+                figment,
+                self.ignore_missing_auth_providers,
+            )
+            .await?;
             check_warnings.extend(extra_warnings);
             check_errors.extend(extra_errors);
         }
@@ -261,6 +274,7 @@ impl Options {
                     &mut rng,
                     provider_id_mappings,
                     &progress,
+                    self.ignore_missing_auth_providers,
                 )
                 .await?;
 
diff --git a/crates/syn2mas/src/migration.rs b/crates/syn2mas/src/migration.rs
index e32bfd6b9..d71e8cd05 100644
--- a/crates/syn2mas/src/migration.rs
+++ b/crates/syn2mas/src/migration.rs
@@ -141,6 +141,7 @@ struct MigrationState {
 /// - An underlying database access error, either to MAS or to Synapse.
 /// - Invalid data in the Synapse database.
 #[expect(clippy::implicit_hasher)]
+#[allow(clippy::too_many_arguments)]
 pub async fn migrate(
     mut synapse: SynapseReader<'_>,
     mas: MasWriter,
@@ -149,6 +150,7 @@ pub async fn migrate(
     rng: &mut impl RngCore,
     provider_id_mapping: std::collections::HashMap<String, Uuid>,
     progress: &Progress,
+    ignore_missing_auth_providers: bool,
 ) -> Result<(), Error> {
     let counts = synapse.count_rows().await.into_synapse("counting users")?;
 
@@ -171,8 +173,15 @@ pub async fn migrate(
     let (mas, state) = migrate_threepids(&mut synapse, mas, rng, state, progress_counter).await?;
 
     let progress_counter = progress.migrating_data(EntityType::ExternalIds, counts.external_ids);
-    let (mas, state) =
-        migrate_external_ids(&mut synapse, mas, rng, state, progress_counter).await?;
+    let (mas, state) = migrate_external_ids(
+        &mut synapse,
+        mas,
+        rng,
+        state,
+        progress_counter,
+        ignore_missing_auth_providers,
+    )
+    .await?;
 
     let progress_counter = progress.migrating_data(
         EntityType::NonRefreshableAccessTokens,
@@ -452,6 +461,7 @@ async fn migrate_external_ids(
     rng: &mut impl RngCore,
     state: MigrationState,
     progress_counter: ProgressCounter,
+    ignore_missing_auth_providers: bool,
 ) -> Result<(MasWriter, MigrationState), Error> {
     let start = Instant::now();
     let progress_counter_ = progress_counter.clone();
@@ -489,6 +499,10 @@ async fn migrate_external_ids(
 
                 let Some(&upstream_provider_id) = state.provider_id_mapping.get(&auth_provider)
                 else {
+                    if ignore_missing_auth_providers {
+                        progress_counter.increment_skipped();
+                        continue;
+                    }
                     return Err(Error::MissingAuthProviderMapping {
                         synapse_id: auth_provider,
                         user: synapse_user_id,
diff --git a/crates/syn2mas/src/synapse_reader/checks.rs b/crates/syn2mas/src/synapse_reader/checks.rs
index aee91b62a..c50f1e9f3 100644
--- a/crates/syn2mas/src/synapse_reader/checks.rs
+++ b/crates/syn2mas/src/synapse_reader/checks.rs
@@ -130,6 +130,11 @@ pub enum CheckWarning {
         "Synapse database contains {num_non_email_3pids} non-email 3PIDs (probably phone numbers), which will be migrated but are not supported by MAS."
     )]
     NonEmailThreepidsInDatabase { num_non_email_3pids: i64 },
+
+    #[error(
+        "Synapse database contains {num_users} users associated to the OpenID Connect or OAuth2 provider '{provider}' but the Synapse configuration does not contain this provider."
+    )]
+    SynapseMissingOAuthProvider { provider: String, num_users: i64 },
 }
 
 /// Check that the Synapse configuration is sane for migration.
@@ -257,6 +262,7 @@ pub async fn synapse_database_check(
     synapse_connection: &mut PgConnection,
     synapse: &Config,
     mas: &Figment,
+    ignore_missing_auth_providers: bool,
 ) -> Result<(Vec<CheckWarning>, Vec<CheckError>), Error> {
     #[derive(FromRow)]
     struct UpstreamOAuthProvider {
@@ -309,10 +315,17 @@ pub async fn synapse_database_check(
             let matching_syn = syn_oauth2.get(&row.auth_provider);
 
             let Some(matching_syn) = matching_syn else {
-                errors.push(CheckError::SynapseMissingOAuthProvider {
-                    provider: row.auth_provider,
-                    num_users: row.num_users,
-                });
+                if ignore_missing_auth_providers {
+                    warnings.push(CheckWarning::SynapseMissingOAuthProvider {
+                        provider: row.auth_provider,
+                        num_users: row.num_users,
+                    });
+                } else {
+                    errors.push(CheckError::SynapseMissingOAuthProvider {
+                        provider: row.auth_provider,
+                        num_users: row.num_users,
+                    });
+                }
                 continue;
             };