Pearl & Dean sold for £1
The Pearl business was founded in 1953 and is the second largest cinema advertising firm in the UK with more than a third of the market.
But STV said the business has been loss-making due to the impact of a screen advertising contract with Vue Cinemas in 2004. That deal, which has required minimum income guarantee payments to Vue, ends in December.
The new owner of Pearl - subject to the approval of STV shareholders - will be Irish businessman Thomas Anderson, who owns Empire Cinemas.
Empire operates 144 screens across 17 cinemas in the UK, including the Empire in London's Leicester Square. The business was formed less than five years ago after picking up cinemas sold in the wake of the mergers of Odeon with UCI and Cineworld with UGC.
STV, which has owned Pearl since 1999, said the disposal meant it would be able to concentrate on its broadcasting and television production operations, which are trading in line with expectations.