The fledgling English actor, who plays Samwell Tarly in the HBO drama, will appear in the BBC fantasy show's fifth series in the autumn.
Bradley will portray a character called Tyr Seward, according to Merlin's official Twitter feed.
His Games of Thrones character Samwell is the companion of Jon Snow (Kit Harington).
Bradley joins Game of Thrones co-star Liam Cunningham, who has also signed up for a role in the series.
Digital Spy exclusively revealed back in April that Cunningham would appear in series five of the family drama.
Executive producer Julian Murphy was quoted in May as saying that talks are under way for a sixth run.