Defensive tackle was one of the Bills' biggest needs entering free agency, with only three players - Adolphus Washington, Marquavius Lewis and Rickey Hatley - under contract at the position for next season.
The Bills desperately needed to improve their defensive line.
Williams, 34, will enter his 13th season in Buffalo after being selected in the fifth round of the 2006 draft.
Lotulelei was drafted by the Panthers in the first round of the 2013 NFL Draft when both Bills coach Sean McDermott and general manager Brandon Beane were employed by the Panthers.
The Buffalo Bills had hoped all along that their leader from the 2017 season, defensive tackle Kyle Williams, would return to them for at least one more season as they chased more than just a spot in the playoffs.
Lotulelei, 28, is a five-year vet of the Carolina Panthers.
The signing will help the Bills along the defensive front which suffered greatly after they traded Marcell Dareus to the Jacksonville Jaguars last season. In his career, he has started 75 of 76 games, with 88 tackles, 11.5 sacks, two forced fumbles and two fumble recoveries. ESPN's Adam Schefter was the first to report Lotulelei's pending agreement. Lotulelei is considered a good run stopper but has been overshadowed by Kawann Short in terms of playmaking ability.