Britain initially qualified only one ride for the event in Pontal, Rio.
The 30-year-old Welshman's inclusion comes after several competitors from other nations suffered crashes in the road race on Saturday and withdrew.
"I'm looking forward to racing again on Wednesday," said Thomas, who also crashed but is fit to race.
"Whilst I'm thrilled to represent my country in a second event in Rio, the circumstances surrounding my inclusion are unfortunate and I'd like to wish all those who withdrew through injury a quick recovery.
"I lost quite a lot of skin in my crash and my hip took a pretty big knock, but other than that my legs are feeling good."
Australia's Richie Porte and Vincenzo Nibali of Italy are among those to have withdrawn following crashes in the road race on Saturday, which left them both with fractures - Nibali a double collarbone break, and Porte a shoulder blade.
Australia's Richie Porte and Vincenzo Nibali of Italy are among those to have withdrawn following crashes in the road race on Saturday, which left them both with fractures - Nibali a double collarbone break, and Porte a shoulder blade.
