The Seattle Seahawks first season in the NFC was actually 1976. The Seahawks and the Tampa Bay Buccaneers were both expansion teams that season, with Tampa playing in the AFC. In 1977, the teams switched conferences.
They joined the NFL in 1976, though in the National Football Conference (NFC), as an expansion team along with the Tampa Bay Buccaneers. In 1977, the Seahawks switched conferences with the Buccaneers and moved to the AFC Western Division and remained there until the 2001 season and returned to the NFC West in 2002.