The 28-year old arrives at the club after being released from SkyBet League Two side Cambridge United.
Williamson started his career at Worthing before linking up with Glenn Hoddle at his academy in Spain.
In 2011, he joined AFC Bournemouth where he featured in their play-off semi-final against Huddersfield Town.
He has also played for Port Vale and Gillingham.
He has been training with the squad recently and also featured in the 2-1 win against AFC Stoneham on Tuesday night.
First-team manager Richard Hill said: “I’m delighted to secure the services of Ben, especially with a list of National League teams after him”.