diff --git a/src/api.rs b/src/api.rs
index ca2f2d377..9d0bafe19 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -27,6 +27,7 @@ use crate::{
 fn make_reqwest_client() -> Client {
     Client::builder()
         .timeout(Duration::from_secs(10))
+        .user_agent(crate::SDK_USER_AGENT)
         .build()
         .expect("Could not build client")
 }
diff --git a/src/connection.rs b/src/connection.rs
index 4237dbab8..9ca88fd7f 100644
--- a/src/connection.rs
+++ b/src/connection.rs
@@ -5,7 +5,7 @@ use std::{borrow::Cow, collections::HashMap, str::FromStr};
 use data_encoding::HEXLOWER;
 use reqwest::{multipart, Client, StatusCode};
 
-use crate::{errors::ApiError, types::BlobId};
+use crate::{errors::ApiError, types::BlobId, SDK_HEADER, SDK_USER_AGENT};
 
 /// Map HTTP response status code to an ApiError if it isn't "200".
 ///
@@ -99,9 +99,10 @@ pub(crate) async fn send_simple(
     // Send request
     log::trace!("Sending HTTP request");
     let res = client
-        .post(&format!("{}/send_simple", endpoint))
+        .post(format!("{}/send_simple", endpoint))
         .form(&params)
         .header("accept", "application/json")
+        .header(SDK_HEADER, SDK_USER_AGENT)
         .send()
         .await?;
     log::trace!("Received HTTP response");
@@ -139,9 +140,10 @@ pub(crate) async fn send_e2e(
     // Send request
     log::trace!("Sending HTTP request");
     let res = client
-        .post(&format!("{}/send_e2e", endpoint))
+        .post(format!("{}/send_e2e", endpoint))
         .form(&params)
         .header("accept", "application/json")
+        .header(SDK_HEADER, SDK_USER_AGENT)
         .send()
         .await?;
     log::trace!("Received HTTP response");
@@ -186,6 +188,7 @@ pub(crate) async fn blob_upload(
         .post(&url)
         .multipart(form)
         .header("accept", "text/plain")
+        .header(SDK_HEADER, SDK_USER_AGENT)
         .send()
         .await?;
     map_response_code(res.status(), Some(ApiError::BadBlob))?;
@@ -209,7 +212,11 @@ pub(crate) async fn blob_download(
     );
 
     // Send request
-    let res = client.get(&url).send().await?;
+    let res = client
+        .get(&url)
+        .header(SDK_HEADER, SDK_USER_AGENT)
+        .send()
+        .await?;
     map_response_code(res.status(), Some(ApiError::BadBlob))?;
 
     // Read response bytes
diff --git a/src/lib.rs b/src/lib.rs
index bdeac8a02..69a8dd5bc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -102,6 +102,10 @@ pub use crate::receive::IncomingMessage;
 
 const MSGAPI_URL: &str = "https://msgapi.threema.ch";
 
+// Custom header for identifying the type of SDK used
+pub(crate) const SDK_HEADER: &str = "x-threema-gateway-sdk";
+pub(crate) const SDK_USER_AGENT: &str = concat!("threema-gateway-rs/", env!("CARGO_PKG_VERSION"));
+
 #[cfg(test)]
 mod tests {
     #[test]
diff --git a/src/lookup.rs b/src/lookup.rs
index 36fde88bc..7a6b121df 100644
--- a/src/lookup.rs
+++ b/src/lookup.rs
@@ -6,7 +6,9 @@ use crypto_box::KEY_SIZE;
 use data_encoding::HEXLOWER_PERMISSIVE;
 use reqwest::Client;
 
-use crate::{connection::map_response_code, errors::ApiError, RecipientKey};
+use crate::{
+    connection::map_response_code, errors::ApiError, RecipientKey, SDK_HEADER, SDK_USER_AGENT,
+};
 
 /// Different ways to look up a Threema ID in the directory.
 #[derive(Debug, PartialEq)]
@@ -134,7 +136,11 @@ pub(crate) async fn lookup_pubkey(
     debug!("Looking up public key for {}", their_id);
 
     // Send request
-    let res = client.get(&url).send().await?;
+    let res = client
+        .get(&url)
+        .header(SDK_HEADER, SDK_USER_AGENT)
+        .send()
+        .await?;
     map_response_code(res.status(), None)?;
 
     // Read response body
@@ -177,7 +183,11 @@ pub(crate) async fn lookup_id(
     debug!("Looking up id key for {}", criterion);
 
     // Send request
-    let res = client.get(&url).send().await?;
+    let res = client
+        .get(&url)
+        .header(SDK_HEADER, SDK_USER_AGENT)
+        .send()
+        .await?;
     map_response_code(res.status(), Some(ApiError::BadHashLength))?;
 
     // Read and return response body
@@ -196,7 +206,11 @@ pub(crate) async fn lookup_credits(
     debug!("Looking up remaining credits");
 
     // Send request
-    let res = client.get(&url).send().await?;
+    let res = client
+        .get(&url)
+        .header(SDK_HEADER, SDK_USER_AGENT)
+        .send()
+        .await?;
     map_response_code(res.status(), None)?;
 
     // Read, parse and return response body
@@ -226,7 +240,11 @@ pub(crate) async fn lookup_capabilities(
     debug!("Looking up capabilities for {}", their_id);
 
     // Send request
-    let res = client.get(&url).send().await?;
+    let res = client
+        .get(&url)
+        .header(SDK_HEADER, SDK_USER_AGENT)
+        .send()
+        .await?;
     map_response_code(res.status(), Some(ApiError::BadHashLength))?;
 
     // Read response body