The former Supernatural star will play a vampire called Rose, Zap2It says.
Rose, who is over 500 years old, is said to be a "tough girl" who is also extremely elegant. Rumours have suggested that she will be a new love interest for Damon (Ian Somerhalder).
However, a source claimed that Rose will also be hardened from being a vampire for so long, saying: "Maybe there's some humanity left in her - have fun finding it."
Cohan is expected to make her debut in the eighth episode of the season. The Vampire Diaries continues on Thursdays at 8/7c on The CW.