South Africa's ambassador to the United States, Roelof "Pik" Botha, was named foreign minister today in a move to indicate the country's continued interest in strenghthening ties with Washington.

The charismatic and politically moderate ambassador, will replace retiring Foreign Minister Hilgard Muller, who is known to have wanted to retire for over a year. The move will be made "within a few months," Prime Minister John Vorster said this afternoon.

The foreign minister post is considered a stepping stone to the premier-ship here.

The move has important implications for South Africa's foreign policy. Botha, 44, who also serves as U. N. ambassdor. has been the key link between South Africa and the United States during the Anglo-American settlement effort in Rhodesia.

Under Botha, the SOuth African embassy did an abruptly about-face in terms of its visibility in Washington and at the United Ntaions. Previously, the embassy had maintained a low profile, but Botha responded sharply and quickly to the usual criticism of South Africa at the world organization and he frequently made his country's point of view known to journalists.

Former Secretary of State Henry A. Kissinger was able to talk Rhodesian Prime Minister Ian Smith into an initial agreement to majority rule only with the assistance of South African pressure. Botha did much of the leg-work in arranging the South African involvement and the two Vorster-Kissinger summits that led to Smith's concessions.

Botha has worked hard during his two years as ambassador in Washington to improve relations and seek support from the United States often seeking meetings with Kissinger. He has already had two interviews with new Secretaty of State Cyrus Vance, one just a few days before Vance was sworn in.

Botha is also considered to be one of those pushing for more rapid change in South Africa, realizing that the outside world has become increasingly critical of his country's race policies.

He has actively backed detente with black Africa, explaining in a 1974 magazine interview. "Being an African state ourselves, our destiny lies in Africa and we are aware of the need to come to an understanding with Africa and with the black Africans living in South Africa."

"Change is inherent in what we do and want to do. We acknowledge that change will come. And I say to you, my government will work towards change; no question about it. We feel it is in our own interests.Certainly its in the interest of everyone in Africa that we should come to an understanding."

Botha is perhaps best known for his hour-long speech at the Security Council in October 1974, during which he admitted that South Africa did have discriminatory laws and practices, and unsavory and reprehensible incidents between blacks and whites did occur.

In the sharp statement, aimed at preventing the explusion of South Africa from the United Nations, he tried to explain:

"We have not waged war against black Africa or against anyone.We were, in fact, the first African nationalists. Black Africans neen not conduct a freedom struggle against my government. Being an African country, we understand African aspirations. We have stolen land from nobody. We have conquered no people. We threaten no one. We have absolutely no designs of aggrandizement."

Two weeks later, however, the General Assembly, by vote of 91-22, with 19 abstentions, suspended South Africa from the session.

Employed by the Foreign Ministry since the age of 21, Botha is considered by many to be South Africa's most effective diplomat. During his relatively brief career, in South African terms, he has held prestigious jobs in the SOuth African legations in WSweden, West Germany, United States and at eight U. N. sessions since 1966.

He also argued South Africa's case for administrative rights over Namibia (Southwest Africa) at the International Court of Justice in 1970 and 1976.

The announcement of his appointment also reflects an interesting development in terms of politics here. Botha represents the verligte (Africakaans for "enlightened") faction in the National Party governement.

It was believed that a verkrampte (Afrikaans for "hard-core")member of the Cabinet would win the important foreign affairs post, which has been hotly contested for over a year.

The appointment clearly is a statement to the outside world that South Africa want s to maintain friendly ties - which Botha has battled to maintain - and that the Vorster government wants to appear to its few remaining allies to be installing moderates in the government.

It may also be an indication of Prime Minister Vorster's thinking about a succesor, for Botha is a man who is as moderate as possible within the context of current government policy. Vorster, did not elaborate on the appointment in his announcement.

No replacement for Botha as ambassador to Washiington or the United Nations has yet been announced.