diff --git a/Cargo.lock b/Cargo.lock
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1266,6 +1266,7 @@
  "clap",
  "comm-lib",
  "derive_more",
+ "grpc_clients",
  "once_cell",
  "reqwest",
  "serde",
diff --git a/services/backup/Cargo.toml b/services/backup/Cargo.toml
--- a/services/backup/Cargo.toml
+++ b/services/backup/Cargo.toml
@@ -20,6 +20,7 @@
   "grpc_clients",
   "crypto"
 ] }
+grpc_clients = { path = "../../shared/grpc_clients" }
 once_cell = { workspace = true }
 tokio = { workspace = true, features = ["rt-multi-thread", "macros"] }
 tokio-stream = { workspace = true }
diff --git a/services/backup/src/error.rs b/services/backup/src/error.rs
--- a/services/backup/src/error.rs
+++ b/services/backup/src/error.rs
@@ -8,6 +8,7 @@
 pub use aws_sdk_dynamodb::Error as DynamoDBError;
 use comm_lib::database::Error as DBError;
 use comm_lib::{auth::AuthServiceError, blob::client::BlobServiceError};
+use grpc_clients::error::Error as IdentityClientError;
 use reqwest::StatusCode;
 use tracing::{error, trace, warn};
 
@@ -16,9 +17,11 @@
 )]
 pub enum BackupError {
   NoBackup,
+  NoUserID,
   BlobError(BlobServiceError),
   AuthError(AuthServiceError),
   DB(comm_lib::database::Error),
+  IdentityClientError(IdentityClientError),
 }
 
 impl From<&BackupError> for actix_web::Error {
@@ -65,6 +68,11 @@
           ErrorInternalServerError("server error")
         }
       },
+      BackupError::IdentityClientError(err) => {
+        warn!("Transient identity error occurred: {err}");
+        ErrorServiceUnavailable("please retry")
+      }
+      BackupError::NoUserID => ErrorBadRequest("bad request"),
     }
   }
 }
diff --git a/services/backup/src/identity/mod.rs b/services/backup/src/identity/mod.rs
new file mode 100644
--- /dev/null
+++ b/services/backup/src/identity/mod.rs
@@ -0,0 +1,51 @@
+use comm_lib::crypto::siwe;
+use grpc_clients::{
+  error::Error,
+  identity::{
+    get_unauthenticated_client, protos::unauth::FindUserIdRequest,
+    protos::unauthenticated::find_user_id_request::Identifier,
+    PlatformMetadata,
+  },
+  tonic::Request,
+};
+
+use crate::config::CONFIG;
+use crate::error::BackupError;
+
+// Identity service gRPC clients require a code version and device type.
+// We can supply some placeholder values for services for the time being, since
+// this metadata is only relevant for devices.
+const PLACEHOLDER_CODE_VERSION: u64 = 0;
+const DEVICE_TYPE: &str = "service";
+
+/// Returns `userID` for both Password and Wallet users.
+pub async fn find_user_id(
+  public_user_identifier: &str,
+) -> Result<String, BackupError> {
+  let mut grpc_client = get_unauthenticated_client(
+    &CONFIG.identity_endpoint,
+    PlatformMetadata::new(PLACEHOLDER_CODE_VERSION, DEVICE_TYPE),
+  )
+  .await?;
+
+  let identifier = if siwe::is_valid_ethereum_address(public_user_identifier) {
+    Identifier::WalletAddress(public_user_identifier.to_string())
+  } else {
+    Identifier::Username(public_user_identifier.to_string())
+  };
+
+  let request = FindUserIdRequest {
+    identifier: Some(identifier),
+  };
+  let request = Request::new(request);
+  let response = grpc_client
+    .find_user_id(request)
+    .await
+    .map_err(Error::GrpcStatus)?
+    .into_inner();
+
+  match response.user_id {
+    Some(user_id) => Ok(user_id),
+    None => Err(BackupError::NoUserID),
+  }
+}
diff --git a/services/backup/src/main.rs b/services/backup/src/main.rs
--- a/services/backup/src/main.rs
+++ b/services/backup/src/main.rs
@@ -8,6 +8,7 @@
 pub mod database;
 pub mod error;
 pub mod http;
+pub mod identity;
 
 // re-export this to be available as crate::CONFIG
 pub use config::CONFIG;