BG Group signed the 20-year contract to supply more than 1.2 million tons of LNG from a project in Queensland, Australia.
"Tokyo Gas is now an important foundation customer for our ground-breaking Queensland Curtis LNG Project," said BG Group Chief Executive Officer Frank Chapman in a statement.
The agreement with BP Group is the company's first fully termed sales agreement with Japan for LNG sourced entirely by coal seam gas.
Tokyo Gas assumes a minority stake in the LNG facility on Curtis Island, near Gladstone in Queensland, Australia.
Japan is the largest importer of LNG in the world, with Tokyo Gas the largest supplier in the capital city.
The financial terms of the deal weren't disclosed. Deliveries of LNG under the terms of the contract begin in 2015.