Financial terms of the deal weren't immediately known, but the contract is expected to be "huge," according to Glazer.
"It's huge for me," Long said after a two-hour workout ended early Sunday night. "You like to know where you stand and I feel really good about obviously their commitment to me. When it turns around, I want to be a part of it."
Long was entering the final year of his six-year, $56.5 million rookie contract. The sixth year of the deal was voided after Long met certain incentives. Although the Rams have plenty of salary-cap space, Long's cap number for 2012 was $18.3 million.
Running back Stephen Jackson said he gave Long "a big hug" when he learned of the new deal.
"I think he was one of the pillars of our defense last year and he's very deserving of what he just signed," Jackson said. "It's actually nice to see some guys drafted and not move. Actually, we keep them around, so I was really stoked for him."
Long has 30.5 sacks in 64 NFL games over four years. Last season was his most productive of his NFL career, as Long tallied 13 sacks and 37 tackles. He led the Rams with 15 quarterback pressures and 16 quarterback hits.
The Associated Press contributed to this report.