The 'Da Vinci's Demons' actor will star in an upcoming episode of the sci-fi series alongside Peter Capaldi - who is yet to make his debut as the Time Lord - and Jenna Coleman, in one of two episodes which have been penned by actor and screenwriter Mark Gatiss, and filming will reportedly begin soon.
Expressing his excitement over the project, Tom wrote on Twitter: "Well that's out then. I've been a massive fan since I was even smaller than Jenna. Very excited. @Markgatiss's script is awesome. #DoctorWho (sic)."
The 32-year-old star's comment came after the official 'Doctor Who' Twitter account confirmed rumours that he'd be joining the cast on a short-term basis.
The post read: "Tom Riley will appear in the new series of Doctor Who, starring in an episode written by Mark Gatiss".
Meanwhile, Gatiss - who has created six episodes of the BBC One series in the past - has confirmed that he's written the scripts for two more episodes but confessed he's unsure as to whether both will feature in the new series, which is expected to air later this year.
He wrote on Twitter: "To clarify. I've been commissioned to write two episodes of 'Doctor Who'. That doesn't mean they'll both necessarily be in this season."
'Sherlock' writers Steven Moffat and Steve Thompson are also among the list of names confirmed to be writing for the new series.