Middlesex have allowed wicketkeeper Adam Rossington to be released from the final year of his contract to sign for Northants on a two-year deal.
Rossington, 21, received permission for a release clause to be activated after playing six Championship games on loan to relegated Northants this summer.
"John Simpson remains our number-one keeper," said Middlesex managing director of cricket Angus Fraser.
"That has not allowed Adam the exposure to first XI cricket he is looking for."
|Rossington's run record in first-class cricket|
|After making his debut in 2010, Rossington made 252 runs in nine appearances for Middlesex, at 19.38.|
|Since making his debut in June, he has so far made 440 runs in six appearances for Northants, at 44.00.|
Fraser added: "It is sad for the club when a product of its own youth system moves to another county.
"Adam is an extremely talented young cricketer. We all hope his career blossoms and he fulfils his potential at Northants."
Rossington's promise proved one of the few bright spots in a miserable summer at Wantage Road as Northants were relegated by a record 91 points, losing 12 of their 16 matches - eight of them by an innings.
He hit 440 runs in his six Championship matches for Northants, as he twice extended his initial July loan move, topping the county's averages at 44.00, including his first Championship century,
"It's never an easy decision to leave the club you have played at since you were six," added Rossington. "But I feel it's the right move for me at this moment in time in order to play first-team cricket on a regular basis."
"Adam has proved a popular player with both fans and team-mates," said a Northants statement. "He is delighted to be joining Northants Cricket and is looking forward to the 2015 season and playing first-team cricket on a regular basis."