The team released the details in a press release on Tuesday.
But the run defense was disgusting last season - they allowed a league-worst 18 rush TDs and a whopping 1,487 rush yards - after the team dealt away Marcell Dareus to the Jacksonville Jaguars in Week 8.
The Bills have now addressed one of the biggest weaknesses on their roster by signing Lotulelei and Williams.
Lotulelei's play has fallen off in recent years, but the familiarity with Bills head coach Sean McDermott and general manager Brandon Beane - who were in Carolina when they drafted Lotulelei - clearly carried a lot of weight. He's listed at 6-foot-3 and 311 pounds and fills an immediate hole on Buffalo's defensive front.
After the Bills' wild-card playoff loss to the Jacksonville Jaguars, Williams said he would take time this offseason before deciding his playing future.
Lotulelei has recorded 141 tackles and 11.5 sacks in his career.
Lotulelei was a five-year starter with Carolina after being selected by the Panthers in the first round of the 2013 draft.
Beane has already completed two trades over the past week in dealing quarterback Tyrod Taylor to Cleveland and sending starting offensive tackle Cordy Glenn to Cincinnati. The Bills have two picks in each of the first three rounds, including the 22nd selection.