The Washington Diplomats will announce today the purchase of defender Robert Iarusci from the Cosmos, filling a major need in Coach Gordon Bradley's 1979 plans.

Larusci, 24, has played on three North American Soccer League championship teams in his three years in the league. As a rookie, he played for the champion Toronto Metros He joined the Cosmos in 1977 and played on their back-to-back champions that year and in 1978.

Last season, Iarusci started 22 of 35 games for the Cosmos and was one of the stars of the Soccer Bowl, assisting on two goals in the Cosmos' 3-1 win over Tampa Bay.

Most important however, as far as Bradley is concerned, is that Iarusci is a native of Toronto and will almost certainly be one of the club's two North American starters required under league rules.

"Robert has proven credentials as a soccer player," Bradley said. "What's more, he's proven that he knows how to win."

Iarusci is the second North Amerian defender the Dips have acquired this offseason. Last month, they purchased Don Dragey from the Rocheste Lancers. One of Bradley's major problems last season was finding two outstanding North Americans, especially after Roy Willner, his only native American starter, injured his knee.

Iarusci, 6 feet and 180 pounds, played for the Canadian national team and in the Canadian league before joining the Metros in 1976. He is expected to start for the Dips.

"Iarusci could start for any club in the league," one NASL official said yesterday, "except the Cosmos."