Shondaland is keeping the details of his role on the show under wraps, according to Hollywood Reporter.
"This has been a thrilling secret to keep!" said Rhimes. "One of my favorite family members is home!"
The series centers on Mireille Enos, played by Alice Vaughan, a private investigator with wealthy clients.
As producer, Rhimes has created a role especially for Knight, Deadline reported.
"Shonda, Betsy (Beers) and I are beyond excited to have T.R. back in the Shondaland family and on The Catch in a role that was created and written especially for him," said Heinberg.
Knight is thrilled about it, as well. "Working with Shonda, Betsy and Allan (Heinberg) again fills me with immense joy," Knight said. "I am heartened to be returning to Shondaland and excited to work with this exceptional cast!"
Knight was an original cast member on Grey's Anatomy and played the role of George O'Malley for five seasons before his character was killed off, Entertainment reported.
He earned an Emmy nomination for the ABC medical drama before moving on to star on The Good Wife.
The Catch will return to ABC mid season.