His opponent, the left-wing economist Andrés Arauz, conceded defeat hours after polls closed on Sunday.
With almost all the ballots counted, Mr Lasso had won 52.4% of the vote compared to Mr Arauz’s 47.6%.
Mr Lasso, a 65-year-old three-time presidential candidate, told supporters he accepted the “challenge of changing our country’s destiny”.
“It’s a day on which all Ecuadoreans have decided their future,” he said.
“They have used their vote to express the need for change and…