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".
Hundreds of vehicles were backed up for miles as travellers were warned by Canadian police to reroute.
While now open for travel from Canada to the US, across the other side, the Michigan Department of Transportation said the bridge remained closed and advised drivers to divert to nearby Port Huron to head into Canada.
In remarks on Monday, Mr Trudeau said Canadians were "shocked and frankly disgusted" by some protesters' behaviour, which has reportedly included vandalism and racial abuse.
The demonstrations have been largely centred on the Canadian capital of Ottawa, where vehicles have gridlocked the city and prompted authorities to declare a state of emergency.
For nearly two weeks, hundreds of lorries have brought the city centre to a standstill, forcing many local businesses to close.
Residents' nerves were also being frayed by constantly blaring air horns. On Monday, an Ottawa judge ruled that the truckers must stop honking their horns for 10 days.
While most of the protest has been peaceful, Ottawa police have said they are concerned about the extremist rhetoric coming from far-right groups at the rally. As well as reported racial and homophobic abuse, Nazi symbols have been displayed and protesters danced on the Tomb of the Unknown Soldier at the National War Memorial.
Ottawa police have said they are investigating more than 60 incidents, including alleged hate crimes and property damage.
"I want to be very clear - we are not intimidated by those who hurl insults and abuse at small business workers and steal food from the homeless," Mr Trudeau told MPs at an emergency debate in Canada's House of Commons on Monday.
"We won't give in to those who fly racist flags, we won't cave to those who engage in vandalism or dishonour the memory of our veterans."