Midfielder Braithwaite began his career with Grimsby’s youth setup and signed his first professional contract with the club last season.
He made his senior debut for the Mariners as a substitute this season, in the 3-1 loss to Derby County in the Papa John’s Trophy.
Braithwaite joins a Seasiders outfit sitting 18th in the Pitching In Northern Premier League Premier Division.
The youngster made his debut for Whitby on Saturday, in a 1-0 win over Atherton Collieries in front of the home support.
He will look to continue his positive start with the club next week when Whitby make the trip to Liversedge.
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