Fred Dean, a starting guard for the Redskins in the Super Bowl, has become the first Washington player to jump to the United States Football League, signing what is believed to be a three-year, guaranteed contract with the Tampa Bay Bandits.

"We made a heck of an offer to Freddie, but he decided to go there," Redskins General Manager Bobby Beathard said last night. "His lawyer (Spencer Kopf of Dallas) said it was ridiculous. We didn't. We appreciate what Fred did for us this last year. But it's his choice."

Dean said, "It was a tough decision but Mr. (John) Bassett (the Bandits' managing general partner) offered me a contract I couldn't refuse. It was worth a whole lot more than what the Redskins were offering."

Dean would not discuss salary, but one source said he would be paid about $125,000 a season, not including this one. However, Kopf said that figure was low, and that his client would also receive a sizeable signing bonus.

Dean also will play for Tampa Bay (10-4) the rest of this season--starting next week against Chicago--at approximately $7,000 per game. The signing will be formally announced in Tampa today.

"Financially it puts me up there with the money some of the best linemen in the NFL make," said Dean.

The Washington Federals held the USFL rights to Dean, but traded them to Tampa Bay for a high draft pick in 1984 and a player to be named later, a team spokesman said.

Dean, 28, earned $70,000 in salary with the Redskins in 1982. He had been one of their more valuable reserves since signing as a free agent in 1978 and played at both guard and tackle in his four years in Washington. He also had a number of recurring injuries.

Dean was a reserve for most of the 1982 season, but finally beat out Mark May as the starting right guard for the final game of the regular season against St. Louis, and started against Detroit and Minnesota in the playoffs. May started the NFC championship game against Dallas, but Dean moved back as the starter against the Miami Dolphins and did extremely well.

Beathard, who signed Dean as a free agent for the Dolphins in 1977, said he believes the Redskins have the players to replace Dean. He mentioned sixth-round draftee Bob Winckler, eighth-rounder Todd Hallstrom and free agent Nathan Newton.

Dean, who became a free agent at the end of the season, and Kopf had been negotiating with the Bandits the last 10 days. Kopf said he began the talks after Dean expressed dissatisfaction with the Redskins' offer, which he said Dean would have been "unintelligent" to accept.

In a rather unusual signing announcement, Dean appeared on a halftime interview of a regional telecast of yesterday's Tampa Bay-Birmingham game wearing a Hog T-shirt and a Redskins baseball cap. He took off that hat and replaced it with one bearing Tampa Bay's logo.