Republican Gov. Lee S. Dreyfus shocked Wisconsin Republicans today with the surprise announcement that he will not seek reelection this year.

Hand-in-hand with his wife, Joyce, at a hastily arranged noon appearance in the state capitol, Dreyfus said he has enjoyed his nearly four years as governor but now wants to return to private life.

The announcement ended weeks of low-key speculation about Dreyfus' intentions. He admitted Thursday that he has had "discussions" with Reagan administration officials about a job in Washington, but did not elaborate. There were rumors here that he was being considered as secretary of education.

But he gave no reason today for the decision other than that it was personal. He has remained popular, and was considered to have a strong chance of retaining his job.

Dreyfus, 55, left on a trip to Milwaukee before reporters could question him.

The result was chaos in the Wisconsin Republican Party, which Dreyfus helped rejuvenate in 1978. "We have no money, no organization, no structure and no candidate," moaned Michael Ellis, a GOP leader in the state assembly.

Leaders of both parties agreed that the race for governor is now up for grabs. Ellis said he expects "the biggest crowd ever" to enter the September primaries, predicting at least six GOP candidates and probably as many Democrats.

Until Dreyfus' decision, Wisconsin Republicans had high hopes of riding his coattails to a majority in both houses of the legislature.

The loss of Dreyfus was seen as a blow to the national GOP as well. Dreyfus, a staunch supporter of President Reagan's economic policies, now joins four other Midwestern Republican governors--James A. Rhodes of Ohio, William G. Milliken of Michigan, Al Quie of Minnesota and Iowa's Robert D. Ray--who have declined to seek another term.

While the state Republican Party is left without a standard-bearer, several prominent state Democrats have been mentioned as contenders for governor. Two earlier announced Democratic candidates, Anthony Earl and James Wood, may be overshadowed if former governor Patrick Lucey runs, a decision he is expected to announce shortly.

Lucey retains a fairly strong power base in the state, even though he resigned the governorship in 1976 to become ambassador to Mexico under Jimmy Carter. Other Democrats who have been mentioned include former governor Martin Schreiber, who took over after Lucey resigned and then lost to Dreyfus in 1978; Madison businessman David Carley, and even Sen. William Proxmire.

Speculation about Dreyfus' future ranged from a job in the media, such as television commentator, to returning to the University of Wisconsin at Stevens Point, where he had been chancellor.