The agency picked up the work after a pitch against a number of undisclosed shops, in a process that was handled by Creativebrief.
Harriman Steel will now be responsible for helping to create a fresh identity for the Watch channel, to help "align it better with UKTV's overall strategy".
The agency will also work on the creation of integrated advertising campaigns to promote the channel and its shows, as well as the production of a new series of idents featuring the new Watch branding.
UKTV first started approaching agencies about the business in May, with pitches taking place in the summer.
Watch was launched by UKTV in October 2008 as a general entertainment channel, using the positioning: "See it, love it, share it."
The channel's programming includes Dancing With The Stars and MasterChef Australia, in addition to classic films and repeats of shows from the BBC archive such as the family history series Who Do You Think You Are?.