"I am joining the race for Presidency," 58-year-old Fonseka told a packed press conference here.
Fonseka will be the joint candidate of opposition parties, including the UNP.
His announcement sets the stage for a showdown with incumbent President Mahinda Rajapaksa, who has called for elections two years ahead of schedule.
Voicing confidence that he will win the polls against Rajapaksa, Fonseka said, "This victory (against the LTTE) does not belong to one family."
He was obviously referring to the Rajapaksa brothers -- Mahinda and Defence Secretary Gotabaya Rajapaksa.
Fonseka also complained that his security has been reduced to 25 guards from a contingent of 600 security personnel.
"If I die in an attack on road, hundreds will die along with me," Fonseka warned.