The 24-year-old from Essex will join Eusebio Unzué's Spanish outfit for the 2013 season after years with Team Sky.
Dowsett also won the British time trial title in 2011 and 2012 and was eighth in this year's world time trial championships.
“This isn’t a decision I have taken lightly, Sky is a fantastic team and I was flattered that they wanted me to stay but I have a new opportunity and a new chapter of my career,” Dowsett, who won a silver medal at the Commonwealth Games, said.
“I would like to thank everybody at Team Sky who have supported me over the last couple of years. I now look forward to an exciting 2013 with Movistar.”
Dowsett, who has two stage wins at the Tour of Britain and a victory at the Tour de Poitou-Charentes on his palmares, joins Eros Capecchi and Sylvester Szymd as new faces at Alejandro Valverde's Movistar team.