Truckers rallying against Canada's Covid vaccine rules had blocked the Ambassador Bridge on Monday night, forcing vehicles to take long detours.
The bridge over the Detroit River is a vital trade link between it and the US.
More than 40,000 people and $323m (£238m) worth of goods cross it daily.
According to police in Michigan and Windsor, protesters had forced authorities to close it shortly before 9 PM on Monday - just hours after Canadian Prime Minister Justin Trudeau had accused the demonstrators of attempting to "blockade our economy".