The rating, which indicates ‘improvement necessary’, should have been displayed at the Blue Boar, Hay-on-Wye, Wales.
It was given to the pub after an inspection last year and was supposed to be displayed in the entrance.
A letter had also been sent to the owners to remind them of their duty to inform customers of the rating.
Nigel Vaughan, prosecuting for Powys County Council, said: "The environmental health regulator tries to get businesses to comply with the regulations but when they fail to do so, we then prosecute.
"It is the law and the reason they are there is to help members of the public."
Owner John Roger Golesworthy has admitted fault, said they were not careful enough and indicated willingness to display the score.
He was ordered to pay a £400 fine, £150 in costs, £200 legal expenses and a £40 victim surcharge.