The actor, best known for playing Holby General's Owen Davies between 2001 and 2005, will be seen in the role of Tony Mullen, a patient of Dr Heston Carter (Owen Brenman).
In a forthcoming episode, Tony arrives at the Mill with wife Sandy (Julia Dalkin) and is told that their chances of having a baby are slim. The couple's struggle to conceive stems back to a childhood accident which saw Sandy being hit by a car.
The instalment, titled 'Past Tense', is scheduled to air on Tuesday, June 1 and will explore how the pair's problems leave Tony in a bitter and angry state.
As well as his Holby role, Moraghan has previously starred in Brookside and Dream Team.
Doctors has been celebrating its tenth anniversary this week. Over the past five days, DS has exclusively revealed the castings of Annette Badland, Susannah York, Alyson Spiro and Madhav Sharma in other guest roles on the serial.