The oil on canvas painting by Kyffin Williams, entitled 'Landscape at Llanaelhaearn,' is dated 1947 and measures 51.1cm by 61cm.
The painting had been on display in a secure room on level five of the Royal Festival Hall since it was received on loan from the Arts Council Collection in November 2013.
The painting was reported stolen after a member of staff at the Southbank Centre found its frame, broken, concealed in a toilet cubicle.
It is believed that the painting was stolen at the end of September this year.
Investigating officer Detective Constable Ray Swan of the Metropolitan Police's Art and Antiques Unit said: “Whoever stole this painting is likely to try to sell it.
“I urge anyone who is offered the painting to report the matter to us immediately, or contact us if you know anything about the theft or the painting’s whereabouts.“
Anyone with information on the whereabouts of the painting should call the Art and Antiques Unit on 020 7230 2150.