Belgium manager Marc Wilmots lost his job after the team's failure to go beyond the quarter-finals at the 2016 European Championship, with Wales defeating them 3-1 in the last eight stage.
Martinez has been out of work since being sacked by Everton late last season following a disappointing English Premier League (EPL) campaign.
"Our new head coach: Roberto Martínez! #welcomeRoberto #tousensemble," the Belgium football federation tweeted.
Martinez was appointed as David Moyes' successor in June 2013 and guided Everton to their best EPL points tally -- 72 -- and a fifth-place finish in his first season in charge.
Martinez smashed the club's transfer record with the 28 million pound signing of Romelu Lukaku from Chelsea in the summer of 2014, but failed to build on the success of his debut campaign at the club, finishing at the 11th spot last season.
Nevertheless, the Belgians boast one of the deepest and most talented squads in international football and were ranked No.1 in the world as recently as November 2015.