President Reagan signed the $98.3 billion tax bill today in the seclusion of his mountaintop ranch in the Santa Barbara Mountains northwest of here with a remarkable, and deliberate, lack of fanfare for such an occasion.

While the bill is considered one of the most significant economic measures of the Reagan administration and one of the president's most important political achievements, the White House made every effort to discourage any publicity about the bill signing. In an unusual move, no photographs were allowed of the signing and the White House refused to release a picture of the event.

Usually, the administration does everything but turn handsprings to draw attention to major bill signings. Last year, when the president was similarly vacationing at his ranch, the entire traveling press corps was bused up the mountain from Santa Barbara to witness the signing of the administration's historic tax cut bill.

But this is a tax increase, not a reduction, and it was pushed through the House with the active assistance of the Democratic leadership over the votes of 89 Republican members. In an election year, the administration does not wish to draw unnecessary attention to either the tax increase or to the division in the Republican Party over the legislation.

Reagan did, however, speed up the signing of the measure in the wake of a lawsuit filed by the Airline Passengers Association contending that the government illegally increased the tax on airplane tickets before the measure was signed.

The Treasury Department ruled that the higher tax could take effect on Sept. 1, even though the bill had not been signed, because it was the clear intent of Congress to raise the taxes on that date. A White House official said that despite this ruling it was determined that "the prudent thing to do was to sign the bill as soon as possible."

Presumably, if the case comes to court the administration will be able to argue that the president signed the bill as soon as he received it. If any refunds were ordered, they presumably would involve only the brief period between Wednesday, when they took effect, and today.

Originally, the president had expected to sign the legislation next week when he returned to Washington. But in the aftermath of the association's lawsuit, administration officials decided to send the bill to Reagan as soon as it was received at the White House. That happened Thursday night. Today, White House transportation office courier John Dreylinger carried the bill to Santa Barbara on a commercial flight.

The irony that Dreylinger's airfare was taxed at the higher rate provided in the unsigned bill was not lost on the White House.

"One thing, if they order any refunds it will only be for the flight out here, not the flight back," joked one White House official.

There was some sensitivity here about giving the impression that Reagan was shying away from being associated with the tax increase he had helped lobby through Congress.

"You can write it that way if you want to, but it ain't so," said White House spokesman Larry Speakes during the daily briefing at the White House vacation headquarters in Santa Barbara.

Speakes, pressed to defend the legislation, said that the president thinks the measure is "an important part of his program."

"He went all out for it," Speakes said. "He won the battle. It was a decisive battle in our efforts to keep our economic program moving, to keep our interest rates coming down."

The White House also took some slight comfort in the August unemployment figures, which some officials had expected to top 10 percent but were 9.8 percent, the same as July.

"Our view of it is that unemployment is remaining stable, although the president regards it as still at an unacceptably high level," Speakes said. "It is a lagging indicator. And nearly all the other economic news is good. We believe that, as the economy continues to improve, we are laying the groundwork for a long-term solution to the rise and fall of unemployment."

Reagan relaxed today on his ranch, but skipped his usual morning horseback ride because of atypical cool and windy weather. This afternoon he played host to the White House staff and press corps at a barbecue at a nearby ranch.

Speakes announced that Reagan would return directly to Washington next Tuesday, and award the Freedom Medal to Philip C. Habib, his special ambassador in the Middle East. Reagan had been scheduled to address a campaign event for Utah Sen. Orrin G. Hatch in Ogden. This has been rescheduled for next Friday.