FAIRFAX, Va. (AP) _ Former Rep. John Jenrette was found guilty Thursday of shoplifting a pair of shoes from a suburban Washington department store.

''I find overwhelmingly, without a doubt, that he is guilty on both charges,'' declared Fairfax County, Va., Circuit Judge Donald B. McDonough after a three and one-half hour trial.

The former Democratic congressman from South Carolina was charged with petty larceny and altering price tags at Marshall's discount department store on Dec. 7.

McDonough gave Jenrette concurrent sentences of 90 days for each of two misdemeanor charges, with 75 days suspended. The judge also fined him $1,000 for each charge, with $500 of each suspended. That left Jenrette, 52, facing 15 days in prison and $1,000 in total fines.

Kenneth Robinson, the attorney who represented Jenrette in the Abscam trial as well as the shoplifting case, said he plans to appeal. In the meantime, Jenrette was freed on personal bond.

In October, 1980, Jenrette was convicted of sharing a $50,000 bribe in the undercover Abscam investigation.

During the trial, Heather McKeever, a security guard at the store, described how she saw Jenrette put on a pair of store shoes and place his own in the bottom row of the store's shoe rack.

Jenrette declined comment after the trial.