West Virginia Gov. Joe Manchin said Carte Goodwin, 36, would fill the rest of Byrd's term, which would end after the 2012 election.
Goodwin is to be sworn in Tuesday. While Byrd was both the oldest and longest-serving senator when he died at 92 on June 28, Goodwin will be the youngest as well as the newest.
Goodwin was Manchin's general counsel from 2005-09 and is in private practice that includes commercial litigation, appellate advocacy and intellectual property, the governor's Web site stated.
"I am truly confident that Carte Goodwin will look out for West Virginia and he will make us proud," Manchin said in the release.
Goodwin told reporters he isn't interested in running for the seat in 2012. It has been rumored that Manchin, who has been governor since 2005, is looking into seeking the seat.
Manchin called Goodwin "fiercely independent."
On one important issue, Goodwin said he opposes the cap-and-trade bill because it would hurt West Virginia's economy.