Victor Vescovo, a retired naval officer, made the unsettling discovery as he descended 10,928 meters to a point in the Pacific Ocean's Mariana Trench - the deepest place on Earth - his expedition said in a statement on Monday.
Vescovo made multiple trips through the trench.
The 'Five Deeps' expedition has already successfully reached the bottom of the Atlantic Ocean, Southern Ocean, Indian Ocean and Pacific Ocean.
Vescovo and his team believe they have discovered four new species of prawn-like crustaceans called amphipods.
Tests by Newcastle University researchers found that sea creatures living in the deepest reaches of the sea had fragments of plastic in their stomachs and muscles.
The team also reportedly found a plastic bag and several candy wrappers.
Vescovo's dive was one of five made by the team between April 28 and May 5, which included the deepest marine salvage operation ever attempted. The last time Challenger Deep had a human visitor was in 2012, when Titanic filmmaker James Cameron set the previous record of 10,908 meters aboard his submersible, Deepsea Challenger.
The team plans to visit the Tonga Trench next before going to the Puerto Rico Trench and then the Molloy Deep in the Arctic Ocean.
Victor Vescovo stands next to a submarine that he took to the deepest point on planet.
To show how deep it is, think about this: If you were to drop Mount Qomolangma, which is more than 8,000 meters tall, into the Mariana Trench, there would still be almost two kilometers of water between the mountain's summit and the surface.
At its core is a 9cm-thick titanium pressure hull that can fit two people, so dives can be performed solo or as a pair. According to the BBC, the pressure at the bottom of the ocean is equal to about 50 jumbo jets piled on top of a person.
It was the third time humans dived to the deepest point in the ocean, known as Challenger Deep.
The expedition's mission is to conduct detailed, sonar mapping missions at the five deepest spots in our oceans.
Once thought to be remote, desolate areas, the deep sea teems with life.