Kuper's retirement means the Broncos will have about $31 million in salary cap space, KCNC-TV's Vic Lombardi reported. The guard's departure also means the longest-tenured Bronco will be kicker Matt Prater.
The Broncos took Kuper in the the fifth round of the 2006 NFL Draft. He started 2007 on special teams before starting in the last 11 games at left guard. Kuper remained a starter up until he dislocated his left ankle in the 2011 regular-season finale.
Kuper eventually came back but wasn't the same. In 2012, he started just five regular-season games and one playoff contest. This past season, Kuper played in four games with one start.
As 2013 wore on, Kuper realized the end of his career was drawing near.
"The season kind of furthered my idea that this would probably be the end for me," Kuper told The Denver Post on Monday. "I didn't feel good all season. I did my best when I was out there. I just wasn't the same player. And I think that's a big part of my decision, not being the same player."