The match will take place at the Rose Bowl in Pasadena on July 23.
United group managing director Richard Arnold said: "Playing LA Galaxy at the world-famous Rose Bowl will be an important part of the team's preparations for the 2014-15 football season.
"The club has been visiting Los Angeles since 1950 and this match provides a great opportunity for us to engage with our fans in this region.
"The popularity of soccer in the US continues to strengthen and Major League Soccer and LA Galaxy have been fundamental to this. Our previous matches against MLS opposition have proved popular with fans and players alike and we are delighted to be returning to the US."