BOSTON — The federal government has committed to acquiring over 300 acres of land into trust for the Mashpee Wampanoag tribe, a critical step as the tribe seeks to build a resort casino.

The surprise approval late Friday marks the first lands to be federally designated for the Cape Cod-based tribe whose ancestors encountered the Pilgrims nearly four centuries ago. Under federal law, a tribe may conduct gambling on trust land if the land is the “initial reservation” of an Indian tribe.

Tribal Chairman Cedric Cromwell called it “truly a glorious, monumental day” for the Mashpee Wampanoag, which became a federally recognized tribe in 2007 and has proposed a casino complex that includes three hotels, an event center, restaurants, retail stores and a water park.

Assistant Secretary of Indian Affairs Kevin Washburn said the government will acquire 170 acres in the town of Mashpee for tribal governmental, cultural and conservation purposes and 151 acres in Taunton for building and operating a gambling facility and resort.

The Mashpee lands have long been owned, controlled or used by the tribe for government administration and cultural and ceremonial events.