diff --git a/homeassistant/components/aws_s3/backup.py b/homeassistant/components/aws_s3/backup.py
index 46585ea7e97..97e2baeec8d 100644
--- a/homeassistant/components/aws_s3/backup.py
+++ b/homeassistant/components/aws_s3/backup.py
@@ -5,7 +5,7 @@ import functools
 import json
 import logging
 from time import time
-from typing import Any
+from typing import Any, cast
 
 from botocore.exceptions import BotoCoreError
 
@@ -189,48 +189,68 @@ class S3BackupAgent(BackupAgent):
         )
         upload_id = multipart_upload["UploadId"]
         try:
-            parts = []
+            parts: list[dict[str, Any]] = []
             part_number = 1
-            buffer_size = 0  # bytes
-            buffer: list[bytes] = []
+            buffer = bytearray()  # bytes buffer to store the data
+            offset = 0  # start index of unread data inside buffer
 
             stream = await open_stream()
             async for chunk in stream:
-                buffer_size += len(chunk)
-                buffer.append(chunk)
+                buffer.extend(chunk)
 
-                # If buffer size meets minimum part size, upload it as a part
-                if buffer_size >= MULTIPART_MIN_PART_SIZE_BYTES:
-                    _LOGGER.debug(
-                        "Uploading part number %d, size %d", part_number, buffer_size
-                    )
-                    part = await self._client.upload_part(
-                        Bucket=self._bucket,
-                        Key=tar_filename,
-                        PartNumber=part_number,
-                        UploadId=upload_id,
-                        Body=b"".join(buffer),
-                    )
-                    parts.append({"PartNumber": part_number, "ETag": part["ETag"]})
-                    part_number += 1
-                    buffer_size = 0
-                    buffer = []
+                # Upload parts of exactly MULTIPART_MIN_PART_SIZE_BYTES to ensure
+                # all non-trailing parts have the same size (defensive implementation)
+                view = memoryview(buffer)
+                try:
+                    while len(buffer) - offset >= MULTIPART_MIN_PART_SIZE_BYTES:
+                        start = offset
+                        end = offset + MULTIPART_MIN_PART_SIZE_BYTES
+                        part_data = view[start:end]
+                        offset = end
+
+                        _LOGGER.debug(
+                            "Uploading part number %d, size %d",
+                            part_number,
+                            len(part_data),
+                        )
+                        part = await cast(Any, self._client).upload_part(
+                            Bucket=self._bucket,
+                            Key=tar_filename,
+                            PartNumber=part_number,
+                            UploadId=upload_id,
+                            Body=part_data.tobytes(),
+                        )
+                        parts.append({"PartNumber": part_number, "ETag": part["ETag"]})
+                        part_number += 1
+                finally:
+                    view.release()
+
+                # Compact the buffer if the consumed offset has grown large enough. This
+                # avoids unnecessary memory copies when compacting after every part upload.
+                if offset and offset >= MULTIPART_MIN_PART_SIZE_BYTES:
+                    buffer = bytearray(buffer[offset:])
+                    offset = 0
 
             # Upload the final buffer as the last part (no minimum size requirement)
-            if buffer:
+            # Offset should be 0 after the last compaction, but we use it as the start
+            # index to be defensive in case the buffer was not compacted.
+            if offset < len(buffer):
+                remaining_data = memoryview(buffer)[offset:]
                 _LOGGER.debug(
-                    "Uploading final part number %d, size %d", part_number, buffer_size
+                    "Uploading final part number %d, size %d",
+                    part_number,
+                    len(remaining_data),
                 )
-                part = await self._client.upload_part(
+                part = await cast(Any, self._client).upload_part(
                     Bucket=self._bucket,
                     Key=tar_filename,
                     PartNumber=part_number,
                     UploadId=upload_id,
-                    Body=b"".join(buffer),
+                    Body=remaining_data.tobytes(),
                 )
                 parts.append({"PartNumber": part_number, "ETag": part["ETag"]})
 
-            await self._client.complete_multipart_upload(
+            await cast(Any, self._client).complete_multipart_upload(
                 Bucket=self._bucket,
                 Key=tar_filename,
                 UploadId=upload_id,