Sarnia Fire and Rescue Services is assisting with a search of the St. Clair River in the area of the Blue Water Bridge.
Witnesses reported hearing a splash and seeing an unresponsive person in the water just after 6 a.m. Thursday.
The Port Huron Police Department responded and discovered an abandoned vehicle with Ontario license plates on the bridge.
Public Education Officer Mike Otis said they've deployed a team to help.
"At 6:20 this morning, Sarnia Fire Rescue Services responded with six firefighters, using Marine One and Marine Two, to assist the American first responders in the search," said Otis.
The Lambton OPP Marine Unit, Point Edward Fire and Rescue and Lambton EMS were also assisting authorities in the United States.
The westbound span of the Blue Water Bridge was closed temporarily and Lambton OPP closed all Highway 402 westbound on ramps from Modeland Road to Front Street.
Traffic resumed across the international span at mid morning while a search of the river continues.
Further information was not immediately available.