The 61-year-old actor will play Don MgHee, former Deputy Commissioner of the Met.
The character will appear in the detective drama's fifth series finale, 'Gently In The Cathedral', written by Peter Flannery.
MgHee is a close friend and mentor of Gently (Martin Shaw) from their days working together at the Met in London.
Diana Quick (Silk) will also appear in the episode as barrister Gitta Bronson, while Shrek: The Musical star Nigel Lindsay will play DI Reese Statham, a cop with a vendetta against Gently.
Morgan Watkins (The Hour) will play DS Kieran Lawson, with Ralph Brown - best known for his role as Danny the drug dealer in 1987 film Withnail & I - cast as Gently's nemesis, career criminal Melvyn Rattigan.
Melanie Clark Pullen will also return for the finale as the estranged wife of Gently's partner John Bacchus (Lee Ingleby).
Inspector George Gently is currently filming on location in Durham and the North East and will be shown on BBC One later in the year.