Westerman scored 29 tries in 128 appearances for the Black and Whites after signing from Castleford in 2010.
The 25-year-old made his England debut against Samoa in 2014, but was left out of the squad for the upcoming series against France and New Zealand.
"Hopefully next year we can win some silverware and compete at the top of the table," he told the club website.
"That's my ambition, it's a good enough club and there's no reason why we shouldn't be contesting for medals.
"When I was younger and with England I worked under [Warrington head coach] Tony Smith and I learnt a lot then, so I am looking forward to learning from him again."
Warrington have signed forward Joe Westerman from Hull FC on a three-year contract for a £150,000 transfer fee.