James Harris has signed a three-year contract with Middlesex after turning down a contract offer to stay with Glamorgan.
Harris, 22, has been on the radar of a number of Division One counties after taking advantage of a clause in his contract that allows him to be released if Glamorgan are not in the top flight.
The Welsh quick is understood to have rejected a deal with both his current county and Nottinghamshire, with Lord's his preferred destination.
After making his first-class debut as a 16-year-old, Harris became the youngest Glamorgan bowler to reach the 100 and 200-wicket milestones.
His departure will come as a major blow to the Dragons, who only avoided picking up the wooden spoon in Division Two with a final round victory over Kent in 2012.
Harris tweeted on Monday: "Very happy to announce that I've signed a contract with Middlesex CCC and will be playing @HomeofCricket next season #veryhappy #excited."
Middlesex boss Angus Fraser told the club's website "Everyone at Middlesex is absolutely thrilled that James has chosen to join the club.
"There were many other counties trying to secure his services and the fact he has chosen Middlesex highlights the progress we are making as a club."
"A couple of years ago we were more concerned about losing our best players than signing the finest young cricketers in the country, and in James we have one of the most exciting young players around.
"From my meetings with him he is a very mature and decent young man. James knows what he wants and we will work hard to help him and the rest of the squad to fulfil their ambitions with Middlesex."