The actor will reprise his role as Raf Di Lucca, despite the fact his character was shot and killed in 2017.
He will return to the hospital when his widow Essie, played by Kaye Wragg, continues to struggle with her cancer diagnosis and searches for answers from a comforting place.
McFadden said: “I’ll always look back on my four years on Holby with such fondness as it was one of the happiest jobs I’ve done and it was a real pleasure to get to wear Raf’s scrubs one last time in order to offer his wife Essie some guidance in such a touching episode.”
The episode will air later this month on BBC One.
The star appeared on the medical drama from 2014 to 2017 and went on to win that year's edition of Strictly Come Dancing after being partnered with Katya Jones.
Holby City went off air in April as filming was halted due to the coronavirus pandemic, but returned to the BBC last month.
Filming on Holby City and its sister series Casualty is thought to be resuming later this month, following in the footsteps of BBC soap EastEnders.
With additional reporting by Press Association