The deal is Brown's first permanent signing and Ashbee will go into the team to face Barnsley on Tuesday night.
Former Tigers skipper Ashbee has played 20 times for Nigel Pearson's side this season and the timing of his move has disappointed his former boss.
Ashbee, who has been at Hull for nine years, handed in his transfer request on Friday after informing Pearson a club had offered him a contract.
That was before Hull's 0-0 draw with Championship leaders QPR on Saturday which leaves them 12th in the Championship.
Preston are bottom of the league with 21 points and are six points from safety.
Lillywhites chairman Maurice Lindsay also revealed the club turned down deadline day offers for Andrew Lonergan, Billy Jones and Sean St Ledger.
Source: BBC Sport