diff --git a/s3transfer/__init__.py b/s3transfer/__init__.py
index 056492cb..d142a7fe 100644
--- a/s3transfer/__init__.py
+++ b/s3transfer/__init__.py
@@ -747,6 +747,7 @@ def upload_file(
             unique_id='s3upload-callback-enable',
         )
         if (
+            self._config.multipart_threshold is not None and
             self._osutil.get_file_size(filename)
             >= self._config.multipart_threshold
         ):
@@ -803,7 +804,7 @@ def download_file(
     def _download_file(
         self, bucket, key, filename, object_size, extra_args, callback
     ):
-        if object_size >= self._config.multipart_threshold:
+        if self._config.multipart_threshold is not None and object_size >= self._config.multipart_threshold:
             self._ranged_download(
                 bucket, key, filename, object_size, extra_args, callback
             )
diff --git a/s3transfer/copies.py b/s3transfer/copies.py
index c2ae9ce0..2d782d70 100644
--- a/s3transfer/copies.py
+++ b/s3transfer/copies.py
@@ -130,7 +130,7 @@ def _submit(
 
         # If it is greater than threshold do a multipart copy, otherwise
         # do a regular copy object.
-        if transfer_future.meta.size < config.multipart_threshold:
+        if config.multipart_threshold is None or transfer_future.meta.size < config.multipart_threshold:
             self._submit_copy_request(
                 client, config, osutil, request_executor, transfer_future
             )
diff --git a/s3transfer/download.py b/s3transfer/download.py
index e1a9cf0e..ba8b8649 100644
--- a/s3transfer/download.py
+++ b/s3transfer/download.py
@@ -346,7 +346,7 @@ def _submit(
         :param bandwidth_limiter: The bandwidth limiter to use when
             downloading streams
         """
-        if transfer_future.meta.size is None:
+        if transfer_future.meta.size is None and config.multipart_threshold is not None:
             # If a size was not provided figure out the size for the
             # user.
             response = client.head_object(
@@ -364,7 +364,7 @@ def _submit(
 
         # If it is greater than threshold do a ranged download, otherwise
         # do a regular GetObject download.
-        if transfer_future.meta.size < config.multipart_threshold:
+        if config.multipart_threshold is None or transfer_future.meta.size < config.multipart_threshold:
             self._submit_download_request(
                 client,
                 config,
diff --git a/s3transfer/processpool.py b/s3transfer/processpool.py
index 318f1e7f..5cf05346 100644
--- a/s3transfer/processpool.py
+++ b/s3transfer/processpool.py
@@ -816,7 +816,7 @@ def _do_run(self):
     def _submit_get_object_jobs(self, download_file_request):
         size = self._get_size(download_file_request)
         temp_filename = self._allocate_temp_file(download_file_request, size)
-        if size < self._transfer_config.multipart_threshold:
+        if self._transfer_config.multiparth_threshold is None or size < self._transfer_config.multipart_threshold:
             self._submit_single_get_object_job(
                 download_file_request, temp_filename
             )
diff --git a/s3transfer/upload.py b/s3transfer/upload.py
index 0347857b..17f2206e 100644
--- a/s3transfer/upload.py
+++ b/s3transfer/upload.py
@@ -245,7 +245,7 @@ def provide_transfer_size(self, transfer_future):
         )
 
     def requires_multipart_upload(self, transfer_future, config):
-        return transfer_future.meta.size >= config.multipart_threshold
+        return config.multipart_threshold is not None and transfer_future.meta.size >= config.multipart_threshold
 
     def get_put_object_body(self, transfer_future):
         # Get a file-like object for the given input
@@ -393,14 +393,14 @@ def provide_transfer_size(self, transfer_future):
     def requires_multipart_upload(self, transfer_future, config):
         # If the user has set the size, we can use that.
         if transfer_future.meta.size is not None:
-            return transfer_future.meta.size >= config.multipart_threshold
+            return config.multipart_threshold is not None and transfer_future.meta.size >= config.multipart_threshold
 
         # This is tricky to determine in this case because we can't know how
         # large the input is. So to figure it out, we read data into memory
         # up until the threshold and compare how much data was actually read
         # against the threshold.
         fileobj = transfer_future.meta.call_args.fileobj
-        threshold = config.multipart_threshold
+        threshold = config.multipart_threshold or 8 * 1024 * 1024
         self._initial_data = self._read(fileobj, threshold, False)
         if len(self._initial_data) < threshold:
             return False