Graham Linehan - creator of the Channel 4 sitcom - announced the news during a Q&A at the German re:publica digital conference.
Chris O'Dowd, Richard Ayoade, Katherine Parkinson and Matt Berry are all expected to return for the new 40-minute episode, which will shoot in three weeks' time, according to Bleeding Cool.
Linehan first announced plans for a final IT Crowd special in December 2012, after previously confirming that the sitcom would not return for a full fifth series.
The comedy - which originally aired between 2006 and 2010 - starred O'Dowd, Ayoade and Parkinson as three staff members of the IT department at the fictional Reynholm Industries.
O'Dowd - who played Roy - revealed last year that he was interested in reprising his role, claiming that the show's stars still "need closure".
"I'd love to do something more with those characters," he admitted. "I don't think we said goodbye to them properly."
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Watch a clip from The IT Crowd below: