A spokesperson for Ryanair said: “After take off cabin crew noticed an acrid smell and standard procedure was followed and the aircraft turned back and landed”.
All 154 passengers disembarked safely at about 0730 BST.
Passenger Andy Patterson said:”There was a slight delay before the flight as they were checking out the engines.
“Slightly after take off there was a distinct smell of fumes in the cabin. A short while later the pilot came over the tannoy and said we would return to Belfast as a precautionary measure”, he added.
A spokesperson from Belfast City Airport said emergency services were at the scene as a precaution.
The source of the smell is being investigated.