Ex-airforce chief and political novice Rumen Radev beat the handpicked nominee of centre-right Prime Minister Boyko Borisov by a whopping margin.
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 today.
Radev swept just over 58 per cent of the vote to parliamentary speaker Tsetska Tsacheva's 35 per cent, according to the exit polls.
More From This Section
Borisov, who was re-elected premier for the second time in 2014, has 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," the burly premier said earlier today.
Radev's clear support for the lifting of EU sanctions on Russia over Ukraine and ambivalent statements about the EU and NATO have prompted analysts to speculate that he could pursue closer ties with Moscow.
"I am convinced that the sanctions do not help but only harm... Russia and the EU countries are equally hurt," the jet fighter pilot said during the campaign.
"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 the incumbent is nonetheless a respected figure and commander-in-chief of the armed forces.
Borisov's resignation could trigger early elections by next spring. The premier's popularity has waned and opinion polls suggest that his GERB party would fail to win an outright majority in early elections.