The species - a type of snailfish of the genus Pseudoliparis - was filmed swimming at 8,336m (27,349ft).
It was filmed by an autonomous "lander" dropped into the Izu-Ogasawara Trench, south of Japan.
The lead scientist said the snailfish could be at, or very close to, the maximum depth any fish can survive.
BBC reports the previous deepest fish observation was made at 8,178m, further south in the Pacific in the Mariana Trench. This discovery therefore beats the depth record by 158m.
"If this record is broken, it would only be by minute increments, potentially by just a few meters," Prof Alan Jamieson told BBC News.
The University of Western Australia deep-sea scientist made a prediction 10 years ago that fish would likely be found as deep as 8,200m to 8,400m. A decade of investigations around the globe has confirmed this.
The juvenile Pseudoliparis was filmed by a camera system attached to a weighted frame released from over the side of a ship, the DSSV Pressure Drop. Bait was added to the frame to attract sea life.
Although a specimen was not caught to fully identify its species type, several fish were trapped slightly higher up in the water column in the nearby Japan Trench at a depth of 8,022m.
These, again, were snailfish, Pseudoliparis belyaevi, and set a record for the deepest fish ever caught.
Snailfish are truly remarkable. There are over 300 species, most of which are actually shallow-water creatures and can be found in river estuaries.
But the snailfish group have also adapted to life in the cold waters of the Arctic and Antarctic, and also under the extreme pressure conditions that exist in the world's deepest trenches.
At 8km down, they are experiencing more than 80 megapascals, or 800 times the pressure at the ocean surface.
Their gelatinous bodies help them survive.
Not having a swim bladder, the gas-filled organ to control buoyancy that is found in many other fish, is an additional advantage.
Likewise, their approach to food - they are suction feeders and consume tiny crustaceans, of which there are many in trenches.