Christian will return to Walford in February to attend the wedding of his sister Jane [played by Laurie Brett] to Ian Beale [Adam Woodyatt].
Theatre star John starred in the BBC One soap for four years from 2008 to 2012, and he returned for Lucy Beale's funeral last year.
John said he was "honoured to be returning" during what is shaping up to be a pretty exciting anniversary week.
Executive producer Dominic Treadwell-Collins said: "How could Jane get married to Ian (again) without her brother there? John Partridge and Christian Clarke are loved by the audience and his EastEnders family, and we are all thrilled to have him back for our anniversary."
There will be many familiar faces come February, as soap bosses are pulling out all the stops to make it a memorable week of TV.
Viewers will see the return of Jo Joyner as Tanya Branning and soap favourite Barbara Windsor as Peggy Mitchell.
Also, Hetti Bywater, Jamie Lomas and Tanya Franks will reprise their roles for a special flashback episode to document the death of Lucy Beale, as the killer will finally be revealed in the anniversary week.
Meanwhile, comedian Richard Blackwood will be making his debut on the soap during the live episode. Richard will be Albert Square's newest villain and he's storyline is set to be "explosive".
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