Bell signed with the team, but did not make the Dolphins out of training camp, and was instead signed to the practice squad for the 2003 season.
In 2004, Bell latched on with the team mostly as a special teamer. He appeared in a total of 13 games and collected 10 tackles. He was sidelined with an ankle injury in December, and was placed on the injured reserve for the seasons last three games.
2005 would see Bell appear in all 16 games for the Dolphins. He made his first career interception in a week nine 23-16 loss to the New England Patriots. He would also force one fumble, recover two, record three sacks and finish the year with 35 tackles. The Dolphins started the season 3-7, but finished on a six game win streak, narrowly missing the playoffs with a 9-7 record.
The Dolphins did not contend in 2006, posting a 6-10 record. Bell finished the season fourth on the team with 70 tackles. He started 11 of 16 games at strong safety. He forced three fumbles, recovered two, collected two sacks and knocked down a team leading 12 passes.
2007 would see the Dolphins post a 1-15 record, as Bell recorded five tackles in a week one loss to the Washington Redskins, 16-13 before injuring his achilles tendon and spending the rest of the year on the injured reserve.
Bell started all 16 games at strong safety in 2008, as the team pulled off an unprecedented return to glory behind Chad Pennington, finishing 11-5. Bell led the team with 120 tackles, also defensing 10 passes, recording one sack, forcing three fumbles and recovering a fourth.
Bell started 15 of 16 games at free safety, and garnered his first (and to date only) Pro Bowl selection. He intercepted three passes for 48 yards, led the team with 113 tackles, defensed nine passes, and collected 1.5 sacks. The Dolphins finished 7-9, missing out on the playoffs.
Miami again posted a 7-9 record in 2010, as Bell led the team for the third year in a row with 101 tackles, starting all 16 games. He forced two fumbles, recovered one, knocked down five passes, intercepted one pass for 21 yards, and finished with 1.5 qb sacks.
In 2011, Bell again started all 16 games for the 6-10 Dolphins. He led the team with 107 tackles. He also got two sacks, one interception, one fumble recovery and four passes defensed.
Bell has one year remaining on his current four year contract. He has appeared in a total of 110 games for Miami, starting 75. He has 561 tackles, 11 sacks, six interceptions, nine forced fumbles, seven fumble recoveries and 44 passes defensed.