President Reagan has pardoned Eugenio Martinez, one of those arrested inside the Watergate office building in the June 17, 1972, burglary of Democratic National Committee headquarters.

Administration officials said Reagan signed the pardon on Wednesday.

Martinez is only the second person to be pardoned in the Watergate scandal. The other was former president Richard M. Nixon, who was never charged with a crime but resigned on Aug. 9, 1974, and was pardoned by his successor, President Ford.

Martinez, 60, was sentenced to one to four years in prison for his role in the burglary that touched off the Watergate scandal. He pleaded guilty to conspiracy, burglary and wiretapping counts, and served 15 months at a minimum-security prison in Florida before being paroled in January, 1974. A pardon would clear his name and allow him to vote.

Martinez, who now works at a Miami car dealership, told ABC last night that the pardon "is the end of an era, of a chapter in my life . . . . After today, it will be over."

Saying that he feels that he was merely following the orders of his government, Martinez said that nonetheless he now will "be more than happy to give a better name to my children and my grandchildren."

Martinez also was convicted of conspiracy for his role in the Sept. 3, 1971, break-in at the office of Dr. Lewis Fielding, the psychiatrist of Pentagon Papers defendant Daniel Ellsberg. But that conviction was reversed on appeal.

Daniel E. Schultz, a lawyer here who says he sought pardons for the four Miami Cubans arrested in the Watergate break-in, said last night that the pardon requests had lain dormant at the Justice Department for several years. Schultz said he had not been told of Reagan's decision and did not know why Martinez' pardon had been approved now.

The White House could offer no further explanation last night. It was announced yesterday, however, that Reagan will make an appearance next Friday before a Cuban-American group in Miami. Reagan's strong stand against communism in Central America in general and against Cuban President Fidel Castro in particular has long made him a popular figure among the Cubans in Miami.

Reagan has been courting Hispanics as part of an effort to lay the groundwork for a possible 1984 reelection campaign.

Before the Watergate break-in, Martinez had long been active in CIA-sponsored covert efforts to overthrow the Castro regime. He maintained throughout the Watergate and Ellsberg break-in cases that he believed he was engaged in legitimate national security operations.

"There was absolutely no question that the Miami Cubans had been hoodwinked and had good reason to believe" they were engaged in legitimate intelligence activities, said Schultz, who said he based the pardon requests on this point and on Martinez' long service to the government in a "covert capacity."

"It was never in my mind to do any wrongdoing," Martinez told U.S. District Court Judge Gerhard A. Gesell at a 1974 sentencing hearing in the Ellsberg case.

There was no word from the administration last night about possible pardons for the other three Cubans arrested at the Democratic National Committee headquarters--Bernard L. Barker, Virgilio Gonzalez and Frank Sturgis.

In 1977, President Carter commuted the sentence of G. Gordon Liddy, the mastermind of the break-in, enabling him to be released from prison after serving four years and four months.

Schultz said he first filed a pardon request for Martinez while Nixon was still in office and resubmitted it in the Ford administration. After Martinez' conviction in the Ellsberg case was reversed on appeal, the pardon request was resubmitted again, he said.

There were seven original defendants in the Watergate break-in case. The others were James McCord Jr. and E. Howard Hunt Jr.