Menchov, Russia's best stage rider of the past decade, winning the Tour of Spain in 2005 and 2007 and the 2009 Giro, missed this year's Italian race with a knee injury.
"Winning the Giro was no doubt the pinnacle of my career," the Katusha rider was quoted as saying by local outlet R-Sport.
"This year I couldn't take part in the Giro because of the injury. The Giro was my main goal for this season, and I understood it was time to end my sporting career."
Neither Menchov nor his Russian team returned calls when contacted by Reuters.
Menchov started his professional career with Spanish outfit Banesto in 2000. Three years later, competing in his first three-week race - the Tour de France - he finished 11th overall and won the white jersey for the best young rider.
In 2005, a year after joining Dutch team Rabobank, Menchov won his first major crown when he was awarded overall victory on the Tour of Spain after Spaniard Roberto Heras was stripped of the title because of a positive test for the banned drug EPO.
Two years later, the big Russian repeated his Vuelta triumph, then won the Giro in 2009.
He was less successful in the Tour de France.
He finished fourth in 2008 but was promoted to third after third-placed Austrian Bernhard Kohl was disqualified for doping.
In the 2010 Tour, Menchov came third overall but ended up in second spot after Spaniard Alberto Contador was stripped of his victory for doping.
- Sports & Recreation
- Tour de France
- Tour of Spain