Washington — By breaking relations with Cuba, Jamaican Prime Minister Edward Seaga has moved a step closer to the United States - and the Reagan administration is smiling approval.
In the process, the Harvard-educated Mr. Seaga has firmed his role as the key figure among Caribbean leaders opposed to Cuba.
Washington sees Jamaica emerging as the linchpin in its strategy to combat Marxism in the Caribbean and to spark economic growth in the region.
''Jamaica is proving a loyal ally,'' says a senior administration official. ''The island has come a long way from the days when Manley (former Prime Minister Michael Manley) was in office. Jamaica was adrift in those days, flirting with Cuba and President Fidel Castro, and hardly an ally of ours. All that is changed now.''
Mr. Seaga, in office for a year, is moving quickly to shore up the new role for his island. Within days of severing ties with Cuba, Mr. Seaga conferred with the governor of Puerto Rico about forming a Jamaica-Puerto Rico economic, educational, and cultural union.
The Seaga government's break with Cuba came Oct. 30, but relations were strained from the time Mr. Seaga took over as prime minister. He immediately sent veteran Cuban ambassador Ulises Estrada packing for home, honoring a campaign promise.
The Jamaican action is the latest in a series of Caribbean and Latin American breaks of ties or reductions of ties with Cuba during the last two years. Colombia and Costa Rica suspended relations with Cuba earlier this year in moves that prompted the Reagan administration to exult that Cuba was increasingly isolated in the hemisphere.
The State Department says it is ''an isolation which Cuba has brought on itself through its policy of interfering in the internal affairs of sovereign nations.''
Washington views the Jamaican move as a good omen for its efforts to create a Caribbean Basin development program to counter Cuban ''adventurism'' in the area.
Public and private spokesmen speak of Jamaica as ''key to Washington's overall strategy.''
The Reagan administration doubled US aid to Jamaica this year to $90 million and supported International Monetary Fund credits of $694 million and commercial bank credits of an additional $71 million - geared to help Seaga bring the economy out of a decade-long slump.