The tunnel complex on the Canadian side was evacuated as a precaution while the Windsor police explosives disposal unit examined the package.
In a statement, Windsor police said the suspicious package was "consistent in structure as being a commercial grade smoke bomb or smoke grenade."
Smoke bombs can emit a thick cloud of smoke if ignited.
Police also seized a pair of brass knuckles, a switchblade and a butterfly knife.
The tunnel reopened by 8:45 p.m. ET, according to the Canada Border Services Agency.
The tunnel bus is also running again after Transit Windsor suspended service.
Police said a vehicle with Michigan licence plates driven by the men entered Canada via the tunnel around 5 p.m.
The two men were sent for secondary inspection and a suspicious package was found in a bag inside the vehicle.
When a vehicle arrives at a land border crossing, the driver must speak to an agent. Should further investigation of the driver or vehicle be required, they are sent to secondary inspection at a larger office, located just beyond the booth.
The tunnel, which connects downtown Windsor with Detroit, can handle as many as 2,000 cars an hour.