The silver Hyundai Elantra was stopped near a hotel where the French delegation is staying, outside the fenced-off security zone.
A police hazardous-materials team, with white suits and gas masks, was seen searching the car.
The arrest was the third related to the increased security around the summit.
Earlier this week, police arrested a man and his wife in north Toronto on explosive and firearms charges.
The Integrated Security Unit, which is overseeing security for the summits, did not say what, if any, direct threat thetwo may have posed, but said there was "no risk to public safety at this time".
Leaders from 20 of the world's most powerful countries are meeting in Toronto, the capital of Ontario province, on June 26 and 27 to discuss economic policy.
Meetings of the G8, the world's richest countries, begin in rural Ontario on June 25.
Large demonstrations are expected in Toronto, as activists campaigning on a variety of causes including the environment and global poverty oppose gatherings of the rich and powerful.
The security costs for hosting the summit are expected to exceed $1bn and a huge metal fence has been erected in the commercial heart of Toronto, disrupting business and everyday life.