An eyewitness said "30ft high flames" were coming out of the roof of a marine engineering workshop after reports of an "explosion" on social media.
North Wales Fire Service had four crews tackling the fire on Anglesey, which started at about 21:10 BST on Thursday.
Ynys Mon MP Albert Owen tweeted there were "no reported injuries" and the area around the Holyhead RNLI station was evacuated as a precaution.
"Emergency services have the matter under control," he added.
Holyhead Port, one of the UK's busiest ports with daily ferries to the Republic of Ireland, said the fire was not affecting its operations.
The Welsh Ambulance Service said it had been made aware of the fire but paramedics were not required.
The fire service confirmed the blaze had been brought under control by 23:30.
For more on the incident, the BBC has more by clicking here.