diff --git a/mangum/handlers/aws_alb.py b/mangum/handlers/aws_alb.py
index 33e0968f..d9933e87 100644
--- a/mangum/handlers/aws_alb.py
+++ b/mangum/handlers/aws_alb.py
@@ -16,6 +16,42 @@ class AwsAlb(AbstractHandler):
 
     TYPE = "AWS_ALB"
 
+    def encode_query_string(self) -> bytes:
+        """
+        Encodes the queryStringParameters.
+        The parameters must be decoded, and then encoded again to prevent double
+        encoding.
+
+        https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html  # noqa: E501
+        "If the query parameters are URL-encoded, the load balancer does not decode
+        them. You must decode them in your Lambda function."
+
+        Issue: https://github.com/jordaneremieff/mangum/issues/178
+        """
+
+        params = self.trigger_event.get("multiValueQueryStringParameters")
+        if not params:
+            params = self.trigger_event.get("queryStringParameters")
+        if not params:
+            return b""  # No query parameters, exit early with an empty byte string.
+
+        # Loop through the query parameters, unquote each key and value and append the
+        # pair as a tuple to the query list. If value is a list or a tuple, loop
+        # through the nested struture and unqote.
+        query = []
+        for key, value in params.items():
+            if isinstance(value, (tuple, list)):
+                for v in value:
+                    query.append(
+                        (urllib.parse.unquote_plus(key), urllib.parse.unquote_plus(v))
+                    )
+            else:
+                query.append(
+                    (urllib.parse.unquote_plus(key), urllib.parse.unquote_plus(value))
+                )
+
+        return urllib.parse.urlencode(query).encode()
+
     @property
     def request(self) -> Request:
         event = self.trigger_event
@@ -27,9 +63,7 @@ def request(self) -> Request:
         source_ip = headers.get("x-forwarded-for", "")
         path = event["path"]
         http_method = event["httpMethod"]
-        query_string = urllib.parse.urlencode(
-            event.get("queryStringParameters", {}), doseq=True
-        ).encode()
+        query_string = self.encode_query_string()
 
         server_name = headers.get("host", "mangum")
         if ":" not in server_name:
diff --git a/tests/handlers/test_aws_alb.py b/tests/handlers/test_aws_alb.py
index d922c37d..6d9335d1 100644
--- a/tests/handlers/test_aws_alb.py
+++ b/tests/handlers/test_aws_alb.py
@@ -15,7 +15,7 @@ def get_mock_aws_alb_event(
         },
         "httpMethod": method,
         "path": path,
-        "queryStringParameters": multi_value_query_parameters
+        "multiValueQueryStringParameters": multi_value_query_parameters
         if multi_value_query_parameters
         else {},
         "headers": {
@@ -52,7 +52,13 @@ def test_aws_alb_basic():
         },
         "httpMethod": "GET",
         "path": "/lambda",
-        "queryStringParameters": {"query": "1234ABCD"},
+        "queryStringParameters": {
+            "q1": "1234ABCD",
+            "q2": "b c",  # not encoded
+            "q3": "b%20c",  # encoded
+            "q4": "/some/path/",  # not encoded
+            "q5": "%2Fsome%2Fpath%2F",  # encoded
+        },
         "headers": {
             "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,"
             "image/webp,image/apng,*/*;q=0.8",
@@ -107,7 +113,7 @@ def test_aws_alb_basic():
         "http_version": "1.1",
         "method": "GET",
         "path": "/lambda",
-        "query_string": b"query=1234ABCD",
+        "query_string": b"q1=1234ABCD&q2=b+c&q3=b+c&q4=%2Fsome%2Fpath%2F&q5=%2Fsome%2Fpath%2F",  # noqa: E501
         "raw_path": None,
         "root_path": "",
         "scheme": "http",