The blaze started at the ICL UK Boulby mine in East Cleveland at 00:30 BST on Wednesday.
The workers, who were 1,100m (3,600 ft) under the sea bed and five miles (8km) from land, were treated for smoke inhalation and later discharged.
Polystyrene blocks are believed to have caught fire in the mine.
More than 100 staff were underground at the time, 11 of whom were temporarily unaccounted for, a spokesman for the mine said.
John Chilton, who has worked as a miner for 29 years and is a representative for Unite union, said an underground fire was "the biggest fear for any miner".
"People are very shaken, but miners are a very strong community and we are gathering together," he said.
The fire was extinguished and HM Inspectorate of Mines said a full investigation into the incident would be carried out.
Mining operations at the site have been temporarily suspended.
One of Europe's deepest mines, the site employs about 1,100 people.
Last year, the company announced it was shedding hundreds of jobs as part of a "new business strategy" to produce fertiliser polysulphate.
Seven workers were taken to hospital after a fire broke out at a potash mine, hundreds of metres below the sea bed.