The two-year prison sentence of former representative John W. Jenrette (D-S.C.) was reduced yesterday by a federal judge here to the 13 months Jenrette has already served.

Jenrette was released last night from the federal prison in Atlanta and is to report today to a halfway house in Florence, S.C., where he must spend nights for the next two months.

After his release, Jenrette, 49, traveled to a political function in Galivants Ferry, S.C., where he said that U.S. District Court Judge John G. Penn called "and said, 'You've been down there [in prison] long enough.' "

Penn also reduced Jenrette's five-year probation to three years, but did not alter the $20,000 fine he imposed for Jenrette's 1980 conviction for conspiracy and accepting a $50,000 bribe in the Abscam investigation into political corruption.

In his order, Penn said Jenrette was scheduled to be sent to the halfway house in July, and would have been released on Sept. 9.

Although Jenrette was convicted in 1980, Penn did not impose Jenrette's sentence for more than three years. Jenrette did not enter prison until April 5, 1985.

One of seven former congressmen convicted in the Abscam operation, Jenrette was videotaped telling an undercover FBI agent, "I've got larceny in my heart."

Penn repeated his earlier criticism of the undercover operation in his order. "It seems unlikely that the defendant would have committed the offense but for the strong inducement by government agents with the help of a convicted con man," he said.