The vote on October 15 set up a runoff, to be held in November, between Alvaro Noboa, a banana tycoon, and opponent Rafael Correa.
"I have accumulated complaints from different sectors that allege a series of crimes, so I must review them," Cecilia Armas told reporters.
Armas said she has up to a year to conclude the investigation.
Final results vote released by Ecuador's Supreme Electoral Tribunal showed Noboa won with 26.8 per cent of the poll, or more than 1.4 million votes. Correa was second with 22.8 percent, wth 217,918 fewer ballots.
The remaining votes were spread among 11 other candidates.
Noboa, a pro-free market, old-style populist who pledges to stake his personal fortune to pull a majority of Ecuadoreans out of poverty, made a surprising surge in the final days of the campaign.
His first-place finish prompted cries of fraud from Correa, an anti-establishment candidate who has threatened to default on foreign debt payments in order to fund social programmes.
Correa, a former finance minister, had been the favourite before the vote.
His party reported finding pre-marked ballots outside of polling places, and alleged that rival parties had infilitrated the tables where the votes were counted and totals entered onto tally sheets.