Mission partner the National Geographic said Cameron had reached the depth of 35 756 feet (10 898m) at 7:52 on Monday local time (21:52 GMT on Sunday) in the Mariana Trench in his specially designed submersible.
Cameron is the first person to make a solo dive to the Pacific Ocean valley known as the Challenger Deep, southwest of Guam. And the last dive of any kind there was a relatively brief two-person team back in 1960.
He spent several hours on the Pacific Ocean sea floor, collecting samples for scientific research and taking still photographs and moving images.
The research vessels Mermaid Sapphire and Barakuda waited for him on the surface during the dive.
"We're now a band of brothers and sisters that have been through this for a while," marine biologist Doug Bartlett told National Geographic from the ship before the dive.
Cameron's goal was to bring back data and specimens from the unexplored territory. He is expected to announce the results of the experiment later.
Upon touchdown, Cameron's first target was a phone booth-like unmanned "lander" dropped into the trench hours before his dive.
The submersible that Cameron designed, a "vertical torpedo" of sorts, had already successfully completed an unpiloted dive on Friday.
Cloaked in perpetual darkness
In 1960, a two-person crew aboard the US Navy submersible Trieste - the only humans to have reached Challenger Deep - spent just 20 minutes on the bottom, but their view was obscured by silt stirred up when they landed.
Because of its extreme depth, the Mariana Trench is cloaked in perpetual darkness and the temperature is just a few degrees above freezing, according to members of the team.
The water pressure at the bottom of the trench is a crushing eight tons per square inch - or about a thousand times the standard atmospheric pressure at sea level. Pressure increases with depth.
Cameron, 57, has been running kilometres a day, practising yoga to increase his flexibility for the dive in the sub's cramped quarters and studying deep-ocean science, physician Joe MacInnis told National Geographic News.
MacInnis is a member of the DeepSea Challenge project, a partnership with the National Geographic Society and Rolex.
Cameron already has 72 dives under his belt, including 12 to film Titanic.
The Mariana Trench is located in the western Pacific east of the Philippines and some 200km east of the Mariana Islands.
The crescent-shaped scar in the Earth's crust measures more than 2 550km long and 69km wide on average.
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