The actress and her professional dance partner Robin Windsor left the competition at the semi-final stage after finishing in the bottom two alongside Denise Van Outen and James Jordan.
Both couples performed their highest-scoring dance from Saturday night in the dance-off, with Riley and Windsor performing an American smooth routine to 'All That Jazz', while Van Outen and Jordan did the tango to 'Roxanne' by The Police.
After two standing ovations, the judges delivered their verdict, with Craig Revel Horwood choosing to save Denise Van Outen, saying: "Purely based on technique, Denise and James."
Darcey Bussell said: "This is so hard but, yes, technically, the ones I'm going to save are Denise and James."
Finally, Bruno Tonioli said: "First I have to say you've both done so well to come this far, great dancing, great entertainment, but I have to go for the best dance, and that was delivered by Denise and James."
The unanimous vote meant that Denise Van Outen will join Louis Smith, Kimberley Walsh and Dani Harmer in next week's grand final.
A tearful Lisa Riley said: "It's been the best time of my entire life, I can't thank you enough. From the costume to the make-up, everybody backstage is just beyond belief, and all the judges, I've had the time of my life. And one person has made that, and that's Robin Windsor. He has just been amazing.
"What you don't all know is having me as a partner is a really, really tough ask and every week, week in week out he's done it… and he's helped me through one of the toughest times of my life. I owe every inch of Strictly to him, he's just amazing."
Robin Windsor added: "I just want to say, from day one this whole experience this year has been absolutely amazing, and Lisa has come out and shown you don't have to be a certain size to dance, and she's inspired so many people around the country. She is one of the loveliest, nicest people I've ever met."
Strictly Come Dancing continues on Saturday (December 22) at 7pm on BBC One.
Photo gallery - Strictly Come Dancing semi-final: