James Fuller has signed a three-year contract with Middlesex.
The move has been confirmed on the same day that Gloucestershire announced that the 24-year-old had rejected their offer of a new deal.
"I'm looking forward to showing Middlesex supporters what I can bring with the white ball," the bowler told the club's official website.
"I'm also looking forward to contributing with the red ball too and hope to get plenty of opportunities to show what I can do in the Championship."
Director of cricket Angus Fraser added: "Middlesex CCC have been looking to sign a fast-bowler with white ball pedigree and in James I believe we have found a bowler that will add extra bite and quality to this area of our cricket.
"I have watched James bowl on numerous occasions and have been impressed with what I have seen. He is a fine athlete with a strong, easy action that allows him to bowl with good pace."
In total, Fuller took 182 wickets for Gloucestershire over the three formats of the game.
