James A. Thomson, who has served as chief executive and president of the nonprofit policy think tank Rand Corp. for more than two decades, on Tuesday announced that he will retire this fall.

Thomson, 66, became Rand’s fourth president in 1989 and has served in the post longer than any of his predecessors. Under his leadership, the Santa Monica-based organization’s annual budget has grown from $91 million to more than $250 million, and its staff has grown from 1,050 to more than 1,700 people in nearly a dozen offices around the world.

“I feel privileged to have led Rand for the past 22 years, and have always said this was the best job in the world for me,” said Thomson, a nuclear physicist who worked on European security and arms control before joining Rand’s research staff in 1981. “It is now time for change and I look forward to helping my successor make a smooth transition.”

Rand will begin looking for a successor immediately, according to Board of Trustees Chairman Paul G. Kaminski, who will lead the search committee.

“James Thomson has been an extraordinary leader who has helped Rand increase the breadth of its world-leading research and expanded its research efforts around the globe,” Kaminski said in a statement. “He leaves a strong legacy and a vital organization for his successor to build upon.”