The Eragon actor will play a footman named Jimmy when the ITV drama returns, Digital Spy has learned.
The 23-year-old is currently starring in horror comedy Love Bite and previously featured in Echo Beach.
Speleers's Downton Abbey casting follows the announcement last month that Cara Theobald and Lucille Sharp will play maids in series three.
War Horse star Matt Milne has also signed on to portray a new footman, while Shirley MacLaine is due to appear as Martha Levinson, the mother of Lady Grantham (Elizabeth McGovern).
Downton Abbey is expected to air on ITV later this year.