Russia's space agency told AFP that the Soyuz capsule carrying the astronauts, who hail from Russia, the United States and Italy, had detached from the ISS and was due to land in Kazakhstan as scheduled at 13.43 GMT.
"We confirm that the Soyuz with the crew has left its dock at the planned time," mission control at Russia's Roscosmos agency said.
The delayed return means that Italian astronaut Samantha Cristoforetti has broken the record for the longest single stay by a woman in space with her 200-day mission on the research outpost.
"So long... and thanks for all the fish!" she wrote on Twitter before starting the journey home, using a quote from Douglas Adams' cult science fiction novel "The Hitchhiker's Guide to the Galaxy."
Russia's troubled space programme has experienced a series of recent failures.
It was forced to delay the return of the astronauts and the departure of their replacements in May after a supply ship crashed back to Earth following a rocket failure.
The next manned mission to the ISS is due to blast off between July 23rd and 25th, launching from Kazakhstan with astronauts from Russia, Japan and the US, Roscosmos said on Tuesday.