Laurent Ballesta was selected as the winner of the competition following the submission of his “enigmatic image” Creation.
The magnificent shot captures camouflage groupers exiting their milky cloud of eggs and sperm in Fakarava, French Polynesia.
Over the past five years, Mr Ballesta and his team returned to the lagoon, diving day and night so they did not miss the annual spawning that only takes place around the full moon in July.
His image was selected from more than 50,000 entries from 95 countries and was named the winner at a virtual awards ceremony at the Natural History Museum in central London.
Writer and editor Rosamund “Roz” Kidman Cox, chairperson of the judging panel, said the photograph captures a magical moment.
“The image works on so many levels. It is surprising, energetic and intriguing, and has an otherworldly beauty,” she said.
“It also captures a magical moment, a truly explosive creation of life, leaving the tail-end of the exodus of eggs hanging for a moment like a symbolic question mark.”
This year marks the 57th exhibition with a record number of submissions portraying nature under pressure.
Breathtaking pictures show cheetahs swimming in a raging river in Kenya, a lynx making a comeback in Spain and a slick of dying herrings in the wake of a fishing boat in Norway.
Meanwhile, 10-year-old Vidyun R Hebbar was named Young Wildlife Photographer of the Year 2021.
His colourful image, Dome home, shows a tent spider as a tuk-tuk passes.
The two winners were chosen from 19 categories in total which aim to celebrate the natural world.
Three new categories were introduced this year, including Oceans The Bigger Picture and Wetlands The Bigger Picture.
A total 100 images from the competition will be on display at the Wildlife Photographer of the Year exhibition at the Natural History Museum.
It opens on October 15 before touring across the UK and internationally.