WASHINGTON (AP) _ Basketball all-star David Robinson must fulfill his obligation to the Navy before he can begin his pro career and thus will not be playing this fall, the secretary of the Navy decided Thursday.

Robinson, an ensign in the Navy now in the middle of his second year of active duty, said he accepted the ruling by Navy Secretary William L. Ball III and looked forward ''to serving the remaining six months on active duty ... with pride.''

A 7-foot-1 All-American while at the U.S. Naval Academy, Robinson played with America's Olympic basketball team this summer. After the Olympics ended, he petitioned Ball to be released from active duty before completing his required two-year service commitment.

In return, Robinson suggested the Navy increase his obligation to the Naval Reserve.

Robinson already holds an eight-year, $26 million contract with the San Antonio Spurs and hoped to begin his pro career this year instead of waiting until next. His two-year commitment to the Navy doesn't expire until May 17, 1989.

Ball, however, decided he was not going to tamper with the current Navy policies regarding active-duty servicemen and professional sports endeavors by providing Robinson with a special exception, said one source, who spoke on condition of anonymity.

The Navy declined to discuss Ball's decision beyond acknowledging the secretary had spoken by telephone with Robinson on Thursday morning.

Robinson, currently assigned to the Navy's Trident submarine base at Kings Bay, Ga., as a civil engineering officer, was not available for direct comment.

But his agent, A. Lee Fentress, released a statement in which Robinson complained about ''inaccurate press reports'' and said he was ''proud to have been able to serve in the Navy over the past 17 months.''

Naval Academy graduates are normally required to serve on active duty for a minimum of five years. Robinson's commitment was cut in 1987, however, by then Navy Secretary John Lehman, in part because the basketball star had grown so tall.

His height prevented him from serving in so-called unrestricted line specialities - such as aviation, submarines or surface warfare - after graduation.

Robinson could not have entered the Naval Academy at his present size, but was 6-foot-7 when he was accepted for admission his senior year in high school.

Lehman, in deciding Robinson's future with the Navy, ruled he should serve two years on active duty, four years in the active reserve and two years in the inactive reserve.