Alonso won F1 titles in 2005 and 2006 while driving for Renault but switched to McLaren for the 2007 season and finished tied for second in drivers’ standings, 1 point behind Ferrari’s Kimi Raikkonen.
"Since I was a boy I had always wanted to drive for McLaren, but sometimes in life things do not work out," said Alonso in a statement. "I continue to believe that McLaren is a great team. Yes, we have had our ups and downs during the season, which has made it extra challenging for all of us, and it is not a secret that I never really felt at home."
The decision for Alonso, who had two years left on his contract, to leave McLaren was said to be mutual.
"He is a great driver but for some reason the combination of Vodafone McLaren Mercedes and Fernando has not really worked out and in the end we reached a stage where none of us could find a way to move forward," said McLaren team chairman Ron Dennis.