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The Dragon capsule landed in the Gulf of Mexico, just off the coast of Panama City, Florida, before 3am local time (7am GMT).
Taking only six-and-a-half hours, the journey back to Earth marked the end of the second astronaut flight for the space exploration company.
The capsule, named Resilience, carried home the astronauts - three of whom were American and one Japanese - having launched from the Kennedy Space Center in November.
SpaceX's Mission Control radioed: "We welcome you back to planet Earth and thanks for flying SpaceX.
"For those of you enrolled in our frequent flyer program, you've earned 68 million miles on this voyage."
Spacecraft commander Mike Hopkins said: "We'll take those miles.
"Are they transferable?"
Hopkins then reported that he could see lights from the recovery boats out of the capsule's window.
The 167-day mission is the longest for US-based astronauts, with the previous record of 84 days set by NASA's Skylab crew back in 1974.
The undocking this morning saw four people return from the space station, leaving behind a further seven, who will stay on the ISS.
NASA astronaut Victor Glover, the capsule's pilot, tweeted after leaving the station: "Earthbound!
"One step closer to family and home!"
Hopkins, Glover, NASA's Shannon Walker and Japan's Soichi Noguchi were supposed to land last Wednesday, but SpaceX changed its plans after high winds made daytime landing difficult.
A rare splashdown in darkness was chosen instead, to take advantage of calm weather.
The capsule was tracked with infrared cameras as it entered the atmosphere, where it looked like a bright star flying through the sky.
The four main parachutes were seen deploying just before splashdown.
The previous predawn splashdown took place in December 1968, and saw Apollo 8 return to the Pacific. It had just completed NASA's first flight to the moon with astronauts.
SpaceX's first crew return was in August, when pleasure boaters swarmed the capsule, posing a safety risk - another benefit of the nighttime splashdown.
When the astronauts boarded the SpaceX recovery ship, astronauts were flown back to shore by helicopter, before flying on a plane to Houston to see their families.
The Resilience capsule will be sent back to Cape Canaveral where it will be prepared for SpaceX's first private crew mission in September.
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