The 48-year-old actress - who was a good friend of Luke's and starred alongside him in 'Beverly Hills, 90210' - is to star in the one-off episode of the CW show to remember her late friend.
Sharing the news on her Instagram account, she wrote: ''I am deeply honored to pay tribute to Luke on Riverdale. The care in which this show takes in honoring his memory is beautiful. He is missed. Today. Tomorrow. Forever. (sic)''
And 'Riverdale's showrunner Roberto Aguirre-Sacasa says Shannen will appear in a ''very emotional, important role''.
He told Entertainment Tonight: ''Luke and Shannen were really, really good friends. Luke had talked to me on and off over the years about trying to find something for Shannen on the show and when we were putting together this special episode, we kind of wanted to include special people from Luke's life. There was a very emotional, important role and we sent the script to Shannen and she read it and loved it and said, 'I want to do it,' so it's about honouring their friendship a little bit. I know she was really moved when she read the script and immediately said yes.''
Roberto had previously confirmed the show was waiting to address Luke's death in season four because they wanted to ''figure out the best way'' to honour him.
He said: ''When Luke passed, we had already broken and written a lot of the end of the season, and we really wanted to ... I mean, it was just devastating. And we wanted to honour Luke, and figure out the best way to handle the character of Fred Andrews in the show, so we sort of said, you know what, let's not squeeze, let's not rush to squeeze something into the finale, or into the last couple of episodes, let's really think about it. Let's tell that story at the beginning of season four.''