The Crew Dragon capsule dubbed Endeavour parachuted into the Pacific Ocean off San Diego after midnight on Thursday, capping a 10-hour-plus descent from the International Space Station and fiery re-entry through Earth's atmosphere.
Their return a few weeks ahead of schedule marked the first time that NASA has cut short the mission of an ISS crew due to a health emergency.
Live infrared video presented in a joint NASA-SpaceX webcast showed deployment of the two sets of parachutes from the nose of the free-falling capsule, slowing its rate of descent to about 25km/h before it gently hit the water.
Moments later, several dolphins were visible swimming near the capsule, their dorsal fins breaking the surface of the ocean, as the spacecraft bobbed gently upright in the water.
In a radio transmission to the SpaceX flight-control center near Los Angeles, Endeavour's commander, NASA astronaut Zena Cardman, 38, was heard saying, "It's good to be home."
Joining her on the flight home were fellow US astronaut Mike Fincke, 58, Japanese astronaut Kimiya Yui, 55, and Russian cosmonaut Oleg Platonov, 39.
They arrived together at the space station following a launch to orbit from Florida in August, and departed Wednesday afternoon on a 10-and-a-half-hour flight home, ending a 167-day mission.
The decision to bring all four members of Crew-11 home early was announced January 8, with NASA Administrator Jared Isaacman saying one of the astronauts faced a "serious medical condition" that required immediate medical attention on the ground.
NASA officials have not identified which of the four crew members was experiencing a medical issue or described its nature, citing privacy concerns.
Fincke, a retired Air Force colonel who was the station's designated commander, and Cardman, a rookie astronaut and geobiologist, had been scheduled to conduct a six-hour-plus spacewalk last week to install hardware outside the station.
The spacewalk was cancelled on January 7 over what NASA then characterised as a "medical concern" with an astronaut.
NASA Chief Health and Medical Officer James Polk later said the medical emergency did not involve "an injury that occurred in the pursuit of operations".
