The 28-year-old former Sunderland and Cardiff City player agreed terms with the north-east side on Tuesday after passing a medical at Victoria Park.
Pools manager Martin Scott said: "He is a proven quality player who has performed well in the Championship and Premiership.
"To get the opportunity to sign him was one that we couldn't overlook."
Scott added: "When Chris Westwood left we just felt that it was an area we needed to strengthen and in my opinion there is nobody better to bring in than Darren Williams."
Williams had been offered a new contract at Ninian Park by former boss Lennie Lawrence, however following his departure and Dave Jones taking over at the south Wales club, the offer was withdrawn.