SÃO PAULO—Brazilian presidential candidate Jair Bolsonaro saw his support jump in a poll carried out after he was stabbed last week at a campaign rally ahead of the October elections.

The poll indicated that Mr. Bolsonaro, who has ridden his tough-on-crime stance to unprecedented support for a far-right candidate in Brazil, has become the clear leader in the race ahead of the first round of the country’s presidential election, which will be held on Oct. 7.

If...