In a slickly-produced video published on Twitter Johnson said he was delighted at the creation of a reserve around the South Atlantic British Overseas Territory of Tristan da Cunha - the world's most remote human settlement.
He used the video to promote the climate change agenda he wants to push under Britain's leadership of the COP26 U.N. Climate Change Conference.
"We have a real chance to push for urgent action on climate change, to protect our planet and all its fantastic biodiversity on land and sea," he said.
"We must make this a reality."
The 687,000 square kilometre (265252.18 square miles) marine reserve will close 90% of waters around the volcanic island to harmful fishing methods, and mining. It takes Britain past a target of protecting 4 million square kilometers of ocean.
Johnson, whose senior adviser quit on Thursday, is dealing with a COVID-19 pandemic that has hit Britain harder than its European peers and the final stages of a Brexit process which threatens its economic ties with its biggest trading partner.
Nevertheless, he has pushed climate change up the agenda following Biden's election to the White House.
When the two men spoke in a congratulatory call after the U.S. presidential election, climate change was mentioned by both sides as an important area of common interest. Biden has also highlighted the environment in calls with other leaders.
It is seen as a way to build a relationship with Biden, whom Johnson has never met, and work around the president-elect's past criticism of Brexit and Johnson's approach to implementing it.
Johnson has invited Biden to attend the COP26 summit.
(Reporting by William James; editing by Sarah Young)
