Mike Royko, whose pungent iconoclasm has made his columns a premier news attraction in Chicago for more than two decades, jumped from Rupert Murdoch's newly acquired Sun-Times to the rival Chicago Tribune today, a move he once vowed he would never make.

The Sun-Times' new publisher, Robert Page, responded immediately, saying, "We plan to protect our legal rights . . . [and] take whatever steps are necessary. I suspect you will find us in court tomorrow."

Page accused the Tribune of "malicious interference in our business" by "recruiting" Royko.

The columnist had stormed out of the Sun-Times Dec. 19 after the Field family, who founded the newspaper, announced it had sold the paper to the Australian press tycoon for $100 million. Embittered because Marshall Field V and his half-brother, Frederick, had rejected a last-minute desperation bid by Chicago civic leaders to keep the paper in local hands, Royko took a leave and made it clear in interviews that he didn't like the idea of working for Murdoch.

Today, Royko met the press again, surrounded by grinning Tribune executive and employes, for the announcement that he had signed on. He conceded that the Tribune had been a frequent target when he wrote for the now-defunct Chicago Daily News, his original paper, and the Sun-Times.

"I've made negative comments about everybody and everything in Chicago," he quipped at a laughter-filled half-hour press conference. "I don't retract anything I've said, and I hope that they've mended their ways."

He said he had had political differences with the Tribune, but added he had had political differences with the Sun-Times as well. "I have had criticisms of the Tribune. But the people running the Tribune now are not the people who were running it then.

"All I want to do is write five columns a week in Chicago. I'm not going to his Murdoch's hometown, or his country--wherever he's from--and try to tell him how to run his business. I've written a column in Chicago for 20 years. I'd like to continue writing a column in Chicago for 20 years. But I don't want to work for him. I don't think it's asking him a lot to let me work at what I do for a living without him hauling me into court."

Royko refused to say how much the Tribune will pay him, but said his earnings will be about the same as at the Sun-Times.

He added, "it's less than Julio Cruz," who just signed with the Chicago White Sox for a reported $800,000 a year. Sources said, though, that Royko reportedly will earn about $250,000 annually.

Asked whether his column will mean new circulation gains for the Tribune, he said, "We'll pick up my sisters . . . I have two sisters who will get it . . ."

And, asked if he thought he was being fair to Murdoch, Royko said, "I have a right to work where I want to work. I don't want to work for him. That's fair . . . When it's all over, a lot of people at the Sun-Times are going to get their walking papers from Mr. Murdoch. I don't know if that's fair."

Royko said Murdoch "publishes frivolous papers" and that he is certain Murdoch will not comply with a letter he signed last year promising to continue publishing the Sun-Times in "substantially" the same form.

Royko's decision is a heavy blow to Murdoch's newest acquisition, which formally became his on Monday, robbing it of one of big city journalism's most trenchant voices. Circulation at the Sun-Times, which lags behind the Tribune by more than 100,000 daily readers, will not be helped by the columnist's departure.

It is certain to fuel what is an almost constant feud between the two papers, whose history of bitter competition dates back to 1941, when Marshall Field III began publishing a paper with a liberal editorial policy as an alternative to the once-archconservative Tribune.

At a brief news conference at the Sun-Times, which is a half-block away from the Tribune, Page said his intention is "to see that Royko fulfills" the contract he has with the Sun-Times, which runs through February 1988. Page said it contains a provision pledging Royko not to compete against the Sun-Times.

Asked what Page had said to him when he handed in the letter, Royko retorted, "He said he wasn't a lawyer and he didn't know if he could accept it. I said I'm not a lawyer either."

Royko asserted at his press conference that "I was free to leave, so I did." That contention centers on a provision in the Newspaper Guild's contract at the Sun-Times that allows union members a 15-day grace period to resign in event a new owner takes over and notifies the Guild unit that the 15-day "window" is open. Royko is a Guild member. The Tribune is a nonunion paper.

Although Murdoch's News America Publishing Inc. has officially become the new owner, Page said the "window" notification to the Guild has not yet been made. Royko takes the view that a front-page news story in the Sun-Times made "clear to the world" that the paper has new owners. "And the Guild is part of the world," he said.

That was notification enough for him and he walked into the Tribune today to offer to work for James D. Squires, the editor. He was hired on the spot to a three-year contract for five columns a week. He said the financial terms are "about as much as" at the Sun-Times.

Royko, 51, won a Pulitzer Prize for commentary in 1972 and is the author of "Boss," a biography of the late mayor Richard J. Daley, and four other books.

Even though the Tribune Tower, a Windy City landmark, is only a few hundred yards from the Sun-Times, it was a long journey for Royko. In all his years in Chicago, he had never set foot inside the rival building. The Tribune was prepared, however: there was a guide on hand to show the columnist around his new soapbox.

Royko's first column is scheduled to appear Thursday on Page 3 of the Tribune. That will be a landmark of a different sort.