The Jaguars received cornerback David Jones and a conditional draft pick in the deal, completed after teams made final cuts to reach the 53-man roster limit.
Nelson was the 21st overall pick in 2007 out of Florida. He had an impressive rookie season -- 82 tackles, five interceptions, two forced fumbles and a sack. Since then, he has posted just two interceptions, and his tackles total has dropped.
Nelson shifted positions in Jacksonville after starting as a roaming safety. He spent time at cornerback and was part of nickel pass defenses. The three-year starter lost his job this season, playing at safety with the reserves during preseason games.
Jones was the New England Patriots' fifth-round draft pick out of Division II Wingate in 2007, and the Bengals claimed him off waivers that year. He was primarily an extra cornerback in three seasons with Cincinnati, starting seven games in 2008 after Johnathan Joseph was hurt.
Jones hasn't intercepted a pass in his NFL career, but he could see playing time in the Jaguars' Sept. 12 season opener against the Denver Broncos. The team is making a last-ditch effort to revamp its struggling secondary and cut safety Gerald Alexander and cornerback Scott Starks on Saturday.
That leaves Anthony Smith, Sean Considine, Tyron Brackenridge and Courtney Greene at safety and a void at nickel back.
The Associated Press contributed to this report.