William A. Royall Jr., a political aide to Virginia Gov. John N. Dalton whose organizing efforts are widely credited with giving Dalton his Republican 1977 landslide victory, resigned yesterday to head a direct-mail company in Richmond.

Royall, 33, a sppecial assistant to the governor, will be replaced by Paul G. Edwards, 40, The Washington Post's Richmond correspondent. Edwards will join Dalton's staff Monday at a salary of $33,000 a year.

Royall said he will become president of North American Marketing Copr., a newly formed subsidiary of the North American Corp., a Richmond insurance holding company.

Royall's announcement was a surprise to many, although he has discussed his wish to enter private business with some reporters during recent months. In his $39.700-a-year job, he was Dalton's top political adviser, press secretary and coordinator of the 2,500 patronage jobs the governor dispenses.

A Republican official said that Royall, who is known to favor Attorney General J. Marshall Coleman for the 1981 Republican gubernatorial nomination, is expected to play a key role in Coleman's expected race. Royall declined yesterday to discuss his specific political plans, but said he would remain active in the state GOP.

Royall was Dalton's 1977 campaign manager, and in 1976 ran former President Ford's campaign in Virginia, the only southern state Ford carried. Before that, he was executive director of the Virginia GOP, a job he took after working for the Republican National Committee in Washington.

Edwards, a 16-year veteran at The Post, where he was a financial reporter before joining the Metropolitan staff in 1968, will be an executive assistant to Dalton, the governor's office announced. CAPTION: Picture 1, WILLIAM A. ROYALL JR. . . . going into private business; Picture 2, PAUL G. EDWARDS . . . newsman to join Dalton staff