Virginia Gov. John N. Dalton has granted a conditional pardon to Robert E. Moore, a quadriplegic who was sentenced in 1977 to 21 years in prison for selling $25 worth of marijuana and sleeping pills.

Dalton's pardon, only his third since entering office nearly two years ago, came in response to state legislation that sharply reduced penalties for selling marijuana for first-time offenders. It also followed widespread publicity over the severity of Moore's sentence, imposed by a court in Virginia's conservative Southside.

Moore's pardon was conditioned on his serving one year of the 21-year sentence, paying a $1,000 fine instead of his original $15,000 fine and submitting to 10 years of supervision by the State Parole Board.

The governor personally explained terms of the pardon to Moore on Dec.4 in a 15-minute meeting in the State Capitol, according to Paul G. Edwards, Dalton's press spokesman.

Moore has spent only 47 days in jail since his 1977 conviction. He has been free on bond pending an appeal of the case, the spokesman said. His appeal was made moot by his acceptance of the pardon, state lawyers said.

Dalton's action does not mean automatic pardons for other Virginia prisoners serving long sentences for the sale of marijuana, Edwards said. Each case will be determined upon its own merits, and only after a prisoner-initiated requested for a pardon, he said.

There are several persons serving sentences in excess of 20 years in Virginia prisons for selling marijuana under an earlier state law that allowed up to 40 years imprisonment for the sale of marijuana, according to Brenda Pega, a researcher for the Secretary of the Commonwealth, the office that processes pardons.

For the 29-year-old Moore, the pardon will mean that he will be eligible for release by Dec. 4, 1980. It will also mean one more agonizing move for the 400-pound Moore, who has been confined to a wheelchair since a 1972 shooting incident that left him without the use of his legs and only partial use of his arms.

Moore now is undergoing intensive medical examination at the Medical College of Virginia in Richmond in preparation for his transfer to Staunton Correctional Center, a medium security facility 115 miles southwest of Washington.

Personnel at Staunton Correctional Center have been taking courses in how to handle quadriplegic prisoners readying themselves for Moore's arrival, according to Wayne J. Farrar, spokesman for the Virginia Department of Corrections.