According to Russian space agency officials, the trio successfully docked the Soyuz-TMA16M spacecraft at 01:33 a.m. (UTC) on Saturday after setting off from the Baikonur cosmodrome in Kazakhstan at 7:42 p.m. (UTC) on Friday.
US astronaut Scott Kelly, 51, and 54-year-old Russian cosmonaut Mikhail Kornienko will spend 342 days on board the 400-kilometer high (250-mile) orbiting outpost.
Kelly and Kornienko, both of whom have flown multiple missions to orbit and have each previously spent about six months on the ISS, will be the first people to spend almost an entire year there.
Second Russian cosmonaut, Gennady Padalka, will return to Earth after six months, however.
The trip will mark the longest amount of time that two people will live continuously on the ISS. During the 1990s, however, four Russian cosmonauts spent between 12 and 14 months aboard the Russian space station Mir.
Mission to Mars
The aim of the time spent onboard the ISS is to enable scientists to gain a wider understanding of the effects on the human body of living in microgravity for longer time periods.
Kelly's twin brother Mark is also taking part in the experiment. The retired US astronaut, who previously flew to the ISS four times, will undergo regular health checks on Earth so doctors can compare the brothers' vital signs.
Speaking ahead of Friday's launch, Kelly said the experiment could prove vital in planning future international missions, including to Mars.
"If we ever go beyond Low Earth orbit again, perhaps to Mars, because of the cost and the complexity it will most likely be an international mission, so I see this as a stepping stone to that."
The US astronaut also said he thought the international partnership was "one of the great success stories of the International Space Station."
In September, the newly-arrived crew will also have a visit from the first space tourist since 2009, British soprano Sarah Brightman. The Phantom of the Opera star who has be training at Star City near Moscow since January is set to sing a new song in space, written by her former husband and composer Andrew Lloyd Webber.
ksb/bk (dpa, AFP)