The grant has gone to organisation Ocean 5, reported the New York Post.
"The sad truth is that less than two percent of our oceans are fully protected. We need to change that now," the 39-year-old star said in a statement.
DiCaprio pledged in June, USD 7 million from his foundation for ocean conservation.
Then, in late July, the 'Wolf of Wall Street' star threw a star-studded gala to benefit the Leonardo DiCaprio Foundation, raising more than USD 25 million for environmental causes. Bono, Jared Leto, Robin Thicke and Cara Delevingne were among those on the guest list.