The white supply vessel detached from the orbiting outpost at 9:23 am (1323 GMT), fired its engines three times and slowly began its journey to Earth.
"Release confirmed," said commentator Rob Navias on NASA TV, noting that separation occurred as the ISS was 256 miles (411 kilometers) above the Earth, passing over just south of Australia.
"Dragon is safely on its way."
A parachute-assisted splashdown is expected off the coast of California around 3 pm (1900 GMT), but will not be broadcast on NASA TV.
The spacecraft is bringing back a host of science experiments, including lab mice that were studied in orbit to see how their bones changed in microgravity.
SpaceX's Dragon is currently the only cargo ship designed to return to Earth intact. The other US commercial supply ship, Orbital ATK's Cygnus cargo carrier, burns up on re-entry to Earth's atmosphere.
The cargo ship arrived April 4 after launching from Cape Canaveral, Florida, with 5,800 pounds (2,600 kilograms) of food, supplies and science experiments to enable the study of thunderstorms, anti-cancer drugs, and technology to remove debris in orbit.
The mission was the 14th for SpaceX under a $1.6 billion contract with NASA to resupply the space station over multiple years.
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