Career diplomat Federic Chapin will become the acting U.S. ambassador to El Salvador, replacing Robert White, who was ousted by the Reagan administration after less than a year on the job, the State Department announced yesterday.

Chapin, who has devoted much of his career to Latin American affairs, was the U.S. ambassador to Ethiopia until last July, when he was expelled by that country's Marxist government.

Since then, Chapin, 51, has worked out of the International Security Affairs Office of the Defense Department as deputy assistant secretary for inter-American affairs.

U.S. officials disclosed on Monday that White had been removed from El Salvador because of his decision to go public with many of his views and recommendations.

Spokesman William Dyess, announcing the decision to send Chapin to El Salvador, also told reporters that "we do have evidence that arms from the outside are reaching the guerrillas in El Salvador." Dyess said the evidence is that some of the arms originated in Cuba and that they are reaching Salvador through the territory of Nicaragua.

A U.S. determination that Nicaragua is aiding the rebellion in El Salvador would require the termination of the American aid program for Nicaragua which was approved by Congress last fall with several strings attached. Dyess said this matter is under review.