It is the first US splashdown under the cover of darkness since NASA's Apollo 8 returned from the moon in 1968.
SpaceX's Dragon capsule parachuted into the Gulf Of Mexico just before 8am BST (3am local time).
NASA's Mike Hopkins, Victor Glover and Shannon Walker and Japan's Soichi Noguchi had been orbiting Earth since November last year and undocked from the International Space Station around six hours prior to entering the atmosphere.
Shortly after the Dragon capsule left the ISS, Glover tweeted "Earthbound", as he was hurtling back towards the planet from orbit.
The 167-day mission is the longest for US-launched astronauts, almost doubling the previous record of 84 days which was set by NASA's final Skylab crew in 1974.
SpaceX is the first private space flight operator to take a crew to and from the ISS for a long mission.
Elon Musk's company launched a test mission early last year with two astronauts on-board, who only spent two months aboard the ISS before returning to earth.
After floating in the Gulf of Mexico off the coast of Florida's panhandle, a team on the boat Go Navigator collected the capsule and lifted it on to its deck, to help the crew disembark safely.
All being well, the team will be taken from the boat via helicopter to NASA's headquarters in Houston, Texas, following medical checks, so that they can be reunited with their families.
Once Go Navigator has docked in Florida, the space capsule will be put on the back of a flat-bed truck and driven to NASA's Cape Canaveral, so it can be checked and assessed.
Despite the night-time arrival of the crew, the coastguard enforced an 11-mile wide restricted zone around the splash site, after a number of boats swarmed the capsule when it returned from its test mission in August, posing safety risks for the team.
The capsule will be refurbished ready for SpaceX's first private crew mission in September, which will see a tech-billionaire travel into orbit for three days, 75 miles above the ISS, alongside competition winners and a doctor from a charity.
SpaceX will then launch its second fully-manned mission to the ISS for NASA in October.
NASA has recently turned to private companies in an effort to end its reliance on Russia for space launches after retiring its shuttle fleet in 2011.
Elon Musk's company began unmanned supply runs to the ISS in 2012, and Boeing is expected to begin its manned missions early next year.