Finland's Antti Vierula is to take over the role from Clayton Green who has stepped up to a senior position within McLaren's Human High Performance programme at the team's technology centre in Woking.
Vierula put Hamilton through his paces last winter ahead of the 2010 season, and has again worked with the 26-year-old during the past couple of months.
Hamilton spent the Christmas period in the mountains of Colorado, working at altitude to maximise cardiovascular efficiency via events such as snow-hiking and cross-country skiing.
Vierula, who has worked at the renowned Kuortane Sports Institute in Finland and coached three athletes to the 2008 Beijing Olympics, will be with Hamilton throughout the 20-race season, the longest in F1 history.
Former athlete Vierula will work closely with Hamilton at the gym and track, co-ordinating his fitness programme and dietary needs.
McLaren Racing managing director Jonathan Neale said: "At McLaren, we're committed to building a team with incredible strength in depth in every department.
"By appointing Antti to oversee Lewis' trackside fitness regime and Clayton to a senior position at the MTC we're convinced we've assembled the most experienced, capable and committed team to oversee Lewis' training requirements."