The 39-year-old, who appeared briefly as Nigel Pearson in the second series and full-time in the third series of the Norfolk-based legal drama, is to play down-on-his-luck kids' entertainer Jesse.
A spokeswoman for the ITV soap said of the casting: "We are delighted that John is joining the show. John is a fantastic comedy actor and is perfect for the part of Jesse."
Thomson begins filming his three episodes next month and will appear on screen towards the end of the year.
Producers hope to lure him back for a longer stint next year.
Thomson has previously appeared in Men Behaving Badly, New Street Law and most notably, Cold Feet.