Rumen Radev swept just over 58 percent of the vote to parliamentary speaker Tsetska Tsacheva's 35 percent
Ex-airforce chief and political novice Rumen Radev swept just over 58 percent of the vote to parliamentary speaker Tsetska Tsacheva's 35 percent, exit polls showed.
Borisov earlier threatened to step down if his pro-European Union candidate failed to win.
"We will not participate in any way in the government if we lose today," Borisov said earlier on Sunday.
Observers say the general's victory might tilt ex-communist Bulgaria, which has long walked a tightrope between Moscow and Brussels, towards Russia's orbit -- a trend seen across eastern and central Europe amid rising euroscepticism.
Nearby Moldova also looked set to elect a pro-Russian president on Sunday.
The straight-laced Tsacheva has failed to sway disgruntled voters seeking to punish the government over its perceived failure to tackle rampant corruption and poverty in the European Union's poorest member state.
"I voted against Borisov because I don't think that he's honest and he hasn't really done anything to improve our lives," said 52-year-old Zora Kardachka, a dry cleaner.
The Bulgarian president's role is largely ceremonial but he or she -- Tsacheva would be the first woman in the position -- is nonetheless a respected figure and commander-in-chief of the armed forces.