The military organisation first appeared on the BBC sci-fi drama in 1968 and has appeared sporadically on the show ever since, briefly becoming a permanent fixture in the 1970s.
UNIT will return to Doctor Who - led by Kate (Jemma Redgrave) - in this Saturday's instalment 'The Power of Three', by Chris Chibnall.
"I loved Jemma Redgrave!" said Smith. "She was graceful, funny and charming and an absolute delight."
He added: "UNIT's return is one for the fans. I think they like cyclical things and we have a good story. It was great to do an episode with UNIT and I hope the true fans like it."
Who showrunner Steven Moffat also promised that Chibnall's script will bring UNIT back with "a clever little twist".
"When I was a kid, and just getting obsessed about the Doctor (still not recovered) it was the Jon Pertwee era, and UNIT was as big a part of the show as the TARDIS itself," said the writer.
"So when Chris asked if he could bring them back, I couldn't say yes fast enough. And being Chris, he gave it a clever little twist which I know the fans are going to love.
Doctor Who continues this Saturday (September 22) on BBC One and BBC America.