Porter Max left the ED in January 2018, but actor Jamie Davis will reprise his role for a one-off special episode in December.
Show bosses are keeping his return storyline under wraps, but according to Digital Spy his comeback will not crossover to his old stomping ground of Casualty.
Jamie played Max from 2013 to 2018, and he left with his wife Zoe Hanna, for a new life in America.
Zoe had already departed, but returned in an attempt to rekindle their relationship.
Initally Max refused, but he surprised her by joining her on the plane and flying off to a new life together.
Max's return comes as Holby's boss, Jane Wallbank, has revealed plans for more crossovers between the show and Casualty.
Back in March, the medical dramas combined for an explosive two-part special when a cyber attack left the hospital in a state of choas.
At the time, executive producer Simon Harper had said the project had been "ambitious and unique" and that it would be a "one off".
However, now Jane has told Digital Spy that there will be more crossovers coming up - but not in the same dramatic way.
"The crossover was a huge achievement and everyone worked so hard, so let's see.
"It's probably not viable to do anything on that scale for a few years yet, simply because of logistics.
"But there is increased and more complex character crossover between the two shows on quite a regular basis now.
"We will be playing the cousin relationship between Kian and Casualty's Marty and their family secrets.
"And one of my favourite Casualty characters, David Hide, will be crossing over next year in a big nursing story."
When the shows combined in March popular characters Connie Beauchamp and Jac Naylor were forced to work together to save lives after the cyber-attack - and it was unsurprisingly fraught with tension.
It was the first time the two casts had appeared together since 2005.
Both Casualty and Holby City are set in Holby City Hospital in the fictional county of Wyvern, in the south-west of England.
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