Yes, they were in the AFC West for 25 seasons before returning to the NFC West in 2002.
That's correct, they 'returned' to the NFC West.
In 1976 the Seahawks and Tampa Bay Buccaneers became the 27th and 28th NFL franchises.
The Seahawks were placed in the NFC West and played 13 of their 14 game schedule against the 13 other teams in the NFC.
Meanwhile, Tampa Bay were placed in the AFC West (which seemed strange) and played 13 of their 14 game schedule against the 13 other teams in the AFC.
And the two new teams played each other in Tampa to complete their 14 game schedule.
The following season (1977) both teams switched Conferences: Tampa Bay went to the NFC Central while Seattle went to the AFC West.
Seattle then played 13 of their 14 game schedule against the 13 other AFC teams while the Bucs played the 13 other NFC teams.
And for their 14th game the two newest teams played each other again, this time in Seattle.
So by the end of the 1977 season the two new franchises - Seattle and Tampa Bay - got to play against every single in the NFL once, and they played each other twice, once in Tampa and once in Seattle. Very nice and done in an orderly fashion.
In 1978 the NFL decided to keep Seattle in the AFC West (Tampa remained in the NFC Central).
And with the league now at 28 teams, 1978 also saw the NFL expand to a 16-game schedule, which is still the case today.
Since then four more teams have joined the NFL:
1995: Carolina Panthers and Jacksonville Jaguars
1996: Baltimore Ravens
-- NOTE that the Cleveland Browns team of players and staff moved to become the Baltimore Ravens, but the city of Cleveland were allowed to keep the colors, logo and history of the Browns franchise. So in actuality the NFL regards the Baltimore Ravens as an expansion team. Cleveland received a replacement team in 1999 and they took over the tradition and history of the Browns franchise that ended after the 1995 season.
2002: Houston Texans
With the addition of Houston, it brought the number to 32 NFL teams.
Before the 2002 season began, the NFL realigned much of the league with a few teams switching divisions based primarily on geography.
Before 2002 the NFC West included two teams that played in the Eastern time zone: Atlanta Falcons and New Orleans Saints.
Those two teams were removed from the division and replaced by the Arizona Cardinals and your Seattle Seahawks, who returned to the NFC West after a 25 year absence.
Note also that when the Seahawks switched conferences in 2002, team officials also went with the wishes of their fans and decided to make a few changes to their colors and uniform, which included changing from silver helmets to the current "Seahawks Blue" helmets.
2002 also saw the team play their first season at Seahawks Stadium (aka QuestCenturyLink Field)