diff --git a/dml/src/lib.rs b/dml/src/lib.rs
index 29091c23d1..42b6fc88ca 100644
--- a/dml/src/lib.rs
+++ b/dml/src/lib.rs
@@ -381,6 +381,7 @@ pub mod test_util {
     pub fn assert_writes_eq(a: &DmlWrite, b: &DmlWrite) {
         assert_eq!(a.namespace, b.namespace);
         assert_eq!(a.meta(), b.meta());
+        assert_eq!(a.partition_key(), b.partition_key());
 
         assert_eq!(a.table_count(), b.table_count());
 
diff --git a/generated_types/protos/influxdata/pbdata/v1/influxdb_pb_data_protocol.proto b/generated_types/protos/influxdata/pbdata/v1/influxdb_pb_data_protocol.proto
index 96df307b00..1e1412ef80 100644
--- a/generated_types/protos/influxdata/pbdata/v1/influxdb_pb_data_protocol.proto
+++ b/generated_types/protos/influxdata/pbdata/v1/influxdb_pb_data_protocol.proto
@@ -5,8 +5,16 @@ package influxdata.pbdata.v1;
 option go_package = "github.com/influxdata/influxdb-pb-data-protocol/golang;influxdbpbdataprotocol";
 
 message DatabaseBatch {
+    // The destination database name / namespace for this write.
     string database_name = 1;
 
+    // An optional partition key for this batch.
+    //
+    // If specified, all batches in this write MUST map to this partition key.
+    // Violating this invariant MAY cause data to become unqueryable, degrade
+    // query performance, or other bad things.
+    string partition_key = 3;
+
     // Table data. Data for a given table may appear in multiple table batches.
     repeated TableBatch table_batches = 2;
 }
diff --git a/mutable_batch_pb/src/encode.rs b/mutable_batch_pb/src/encode.rs
index f913801ffc..e296631332 100644
--- a/mutable_batch_pb/src/encode.rs
+++ b/mutable_batch_pb/src/encode.rs
@@ -19,6 +19,10 @@ pub fn encode_write(db_name: &str, write: &DmlWrite) -> DatabaseBatch {
             .tables()
             .map(|(table_name, batch)| encode_batch(table_name, batch))
             .collect(),
+        partition_key: write
+            .partition_key()
+            .map(ToString::to_string)
+            .unwrap_or_default(),
     }
 }
 
diff --git a/router/src/server/grpc.rs b/router/src/server/grpc.rs
index aefe655806..4fd9ef5c33 100644
--- a/router/src/server/grpc.rs
+++ b/router/src/server/grpc.rs
@@ -265,6 +265,7 @@ mod tests {
             database_batch: Some(DatabaseBatch {
                 database_name: "".to_owned(),
                 table_batches: vec![],
+                partition_key: Default::default(),
             }),
         };
 
@@ -287,6 +288,26 @@ mod tests {
             database_batch: Some(DatabaseBatch {
                 database_name: "bananas".to_owned(),
                 table_batches: vec![],
+                partition_key: Default::default(),
+            }),
+        };
+
+        grpc.write(Request::new(req))
+            .await
+            .expect("rpc request should succeed");
+    }
+
+    #[tokio::test]
+    async fn test_write_ok_with_partition_key() {
+        let metrics = Arc::new(metric::Registry::default());
+        let handler = Arc::new(MockDmlHandler::default().with_write_return([Ok(summary())]));
+        let grpc = super::WriteService::new(Arc::clone(&handler), &metrics);
+
+        let req = WriteRequest {
+            database_batch: Some(DatabaseBatch {
+                database_name: "bananas".to_owned(),
+                table_batches: vec![],
+                partition_key: "platanos".to_owned(),
             }),
         };
 
@@ -308,6 +329,7 @@ mod tests {
             database_batch: Some(DatabaseBatch {
                 database_name: "bananas".to_owned(),
                 table_batches: vec![],
+                partition_key: Default::default(),
             }),
         };
 
diff --git a/test_helpers_end_to_end/src/client.rs b/test_helpers_end_to_end/src/client.rs
index 0d3b6fbdad..07df0f9222 100644
--- a/test_helpers_end_to_end/src/client.rs
+++ b/test_helpers_end_to_end/src/client.rs
@@ -50,6 +50,7 @@ pub async fn write_to_router_grpc(
         database_batch: Some(DatabaseBatch {
             database_name: namespace.into(),
             table_batches,
+            partition_key: Default::default(),
         }),
     };
 
diff --git a/write_buffer/src/codec.rs b/write_buffer/src/codec.rs
index 390b817f60..06209cdb8b 100644
--- a/write_buffer/src/codec.rs
+++ b/write_buffer/src/codec.rs
@@ -1,7 +1,7 @@
 //! Encode/Decode for messages
 
 use crate::core::WriteBufferError;
-use data_types::{NonEmptyString, Sequence};
+use data_types::{NonEmptyString, PartitionKey, Sequence};
 use dml::{DmlDelete, DmlMeta, DmlOperation, DmlWrite};
 use generated_types::{
     google::FromOptionalField,
@@ -193,11 +193,16 @@ pub fn decode(
                         ))
                     })?;
 
+                    let partition_key = if write.partition_key.is_empty() {
+                        None
+                    } else {
+                        Some(PartitionKey::from(write.partition_key))
+                    };
+
                     Ok(DmlOperation::Write(DmlWrite::new(
                         headers.namespace,
                         tables,
-                        // TODO(3603): propagate partition key through kafka
-                        None,
+                        partition_key,
                         meta,
                     )))
                 }