London-born Berhalter joins the second-tier English side from Major League Soccer's Vancouver Whitecaps, whom he helped reach the MLS Cup final last season with 16 goal involvements.
The 25-year-old played in all five of the USA's World Cup games, providing a goal and an assist in the country's group-stage loss to Turkey.
Berhalter, who scored six goals and added seven assists in MLS this season, is reunited with childhood friend and compatriot Aidan Morris, with whom he spent time at Columbus Crew and with the US men's national team.
Berhalter said he has been watching Middlesbrough closely over the past two years and "kind of fell in love with the club."
