Martinez, 46, reached an agreement in principle with the Belgian football federation three weeks ago, according to La Derniere Heure newspaper.
The Spaniard's contract was due to run out after this year's European Championship, which was pushed back until 2021 due to the coronavirus pandemic.
Reports indicate Martinez, who took Belgium to third place at the 2018 World Cup, would double his current annual salary of 1.2 million euros ($1.33 million).
The former Everton, Wigan and Swansea manager replaced Marc Wilmots as Belgium boss in August 2016.
bnl/mw/nr