Officers patrolling the motorway stopped the southbound driver at Lymm, where they searched him and his vehicle and found £40,000 in cash.
After discovering the bundle of notes (pictured), officers quizzed the man on where it came from and why he was carrying it around with him.
But he was unable to explain why he was in possession of such a large amount of money.
The £40,000 has been seized whilst officers make further enquiries. No arrests have been made.
They have not said why the driver was pulled over, but the officers were on patrol as ANPR Interceptors whose vehicles are equipped with digital technology which automatically detects vehicle registrations and alerts police to those suspected of committing crime.
Police officers use this information to intercept and stop a vehicle, check it for evidence and, where necessary and appropriate, make arrests.
A police spokesman said: "We stopped a vehicle on the M6 southbound at Lymm - the driver was unable to explain why he was in possession of £40,000 cash - this was seized and further enquiries are ongoing."