Navy Secretary James H. Webb Jr. has approved a Thursday retirement date for Rear Adm. John M. Poindexter, who resigned as national security adviser during the Iran-contra scandal, but deferred a recommendation on his promotion to three-star rank, the Navy announced yesterday.

Webb deferred action on Poindexter's request to retire as a vice admiral "pending resolution of the investigation now being conducted by the independent counsel" Lawrence E. Walsh, the Navy said in a brief statement.

Walsh is investigating Poindexter's role in the trade to Iran of arms for hostages and the diversion of arms-sale profits to aid the Nicaraguan contras.

Poindexter, 51, asked to retire with the three-star rank he held while serving as President Reagan's national security adviser from 1985 until his resignation Nov. 25.

He was dropped to two-star rank when he returned to the Defense Department as a special assistant to the chief of naval operations, a job not designated for three-star rank.

Retirement pay for a three-star admiral is $253 a year more than that of a two-star officer, according to a Navy spokesman. The annual retirement pay of a two-star admiral is $50,508, a Pentagon spokesman said.

When Poindexter asked to retire late last month, a friend said he wanted to leave the service "with his head held high." Friends also said Poindexter wanted to leave before the Iran-contra investigation ended.

Poindexter testified during congressional hearings this summer that he hid details about diversion of arms-sale profits to aid the contras from Reagan in order to protect the president.

He also said he destroyed a presidential document authorizing the sales as an arms-for-hostage deal because of potential political embarrassment.

Although military officers usually are required to serve in a rank for at least three years before retiring at that level, the requirement is waived in many cases with the approval of the Senate and the president.

Some Navy officials said they believed that a request for a waiver in Poindexter's case would have met strong congressional resistance because of the ongoing investigation.