The 36-year-old Millar revealed that his 17th year as a professional will be his last one.
"It's been quite organic really. It's been one of those things where you always think it's going to be a definitive moment, but it hasn't. It's kind of crept up and I'm suddenly realising that it's time."
Millar won his career fourth individual Tour de France stage last season.
He will compete in The Giro d'Italia and the Tour de France. He could also represent Scotland at the Commonwealth Games on home turf in Glasgow.
"I can still be on top of the game, I know that, but it's a lot harder for me now," he said. "I know I can get that out of me with the motivation knowing it's my last year."