Rodney Marsh banged in a 20-foot goal with 1:41 remaining in the game to lift the Tampa Bay Rowdies to a 1-0 victory over the Washington Diplomats in a North American Soccer League contest before 9,099 yesterday at RFK Stadium.

The Dips had just missed a shot at their end and a Rowdy defender cleared the ball out to Marsh. The sometimes moody but talented mid-fielder and team leader slowly began his trek up the field, surprisingly going unchallenged by Washington.

The Dips' defensive unit, which otherwise turned in a fine effort, dropped back while the 33-year-old Marsh approached the Diplomat penalty area.

Marsh, who said his coach, Eddie Firmani, shouted out the time remaining, stopped just outside the 18-foot penalty stripe and connected on a left-foot attempt. Washington goal-keeper Eric Martin never had a chance as the ball curved away from him and sailed into the left side of the net.

"That was our game plan, to score in the last two minutes," grinned Marsh, selected as the game's MVP. "Washington played good defense and I just shot through a crowd of players."

Marsh and his offensive-minded teammates had not had much luck previously against the Dips' aggressive defense.

Tampa Bay outshot Washington, 20-10, but the Dips' defense, led by Roy Lester and Kenny Hill, and helped by several diving saves by Martin, kept the visitors scoreless.

"Our defense played well and will get better," said Diplomat coach Dennis Viollet. "We had a couple of chances we could have put away and didn't. I thought we'd win after we controlled the game the second half."

The Dips had their share of opportunities, particularly in the first half and in the opening minutes of the second half.

At one point, Leroy DeLeon made a nice pass to Gary Darrell, who was coming across the center of the Rowdies' penalty area. But Darrell could not get any power on his shot. The ball dribbled to Tampa Bay goalie Paul Hammond.

Washington has had a number of offensive problems in its early games.

"They created a few chances but didn't take advantage of them," said Tampa Bay defender Farrukh Quraishi, who waged a game-long battle with Washington forward Jimmy Redfern.

The Rowdies could not complain about their lack of chances, either. Derek Smethurst, the NASL's second-leading scorer last year with 45 points, had two perfect opportunities in the first half and two more open shots after intermission. The striker, who has been bothered by a pulled hamstring, had Martin one-on-one after faking out a Washington defender. But Martin guessed right on Smethurst's 10-foot attempt and smothered the ball at the goal line.

Seconds later, Lester headed away a Smethurst attempt in front of the Dips' goal.

Tampa Bay exhibited excellent poise and ball control in the first half. Its overall quickness and short passing attack consistently set up good shot opportunities. But Washington , which has now split six games over a three-year period with the Rowdies, stopped each thrust until Marsh's game-winner.

Washington, which will play four of its next five games on the road, remains in last place in the Eastern Division with a 1-2 record and nine bonus points.