TOKYO — Seven & I Holdings, the Japanese retail giant that owns the 7-Eleven convenience store chain, said Thursday it had agreed to buy the Sunoco chain of gas stations for $3.3 billion, accelerating its expansion in the United States.

The acquisition is believed to be the largest ever for Seven & I and will add Sunoco’s more than 1,000 outlets — with their gas pumps and the convenience stores that operate alongside them, catering to motorists — to its growing portfolio of American businesses.

In addition to 7-Eleven — a business it took over in the early 1990s after initially licensing the brand for use in Japan — Seven & I has been buying up other American retailers. In a smaller deal in June, for instance, Seven & I bought 79 gas stations and convenience stores in California and Wyoming from CST Brands.

Seven & I will purchase the stores from Sunoco LP, which is also a major fuel supplier. The deal for Sunoco’s retail operation, which is concentrated in the Eastern half of the United States, includes an agreement under which Seven & I will continue to buy gasoline from Sunoco for 15 years.