He started in the Baltimore Ravens' season-opener at the Cincinnati Bengals and recorded eight combined tackles, deflected a pass, and intercepted a pass by quarterback Andy Dalton during a 20–0 victory. On October 26, 2017, Mosley made four combined tackles, a pass deflection, was credited with half a sack, and returned an interception for a touchdown during a 40–0 victory on "Thursday Night Football" against the Miami Dolphins in Week 8. Mosley intercepted a pass by Dolphins' quarterback Matt Moore, that was initially intended for wide receiver Jarvis Landry, and returned it for a 63-yard touchdown in the fourth quarter. The touchdown marked his first career pick six. In Week 14, he collected a season-high 14 combined tackles (nine solo) and made a pass deflection in the Ravens' 39–38 loss at the Pittsburgh Steelers. On December 19, 2017, it was announced that Mosley was voted to the 2018 Pro Bowl which marked his third Pro Bowl selection of his career. He started in all 16 games in 2017 and recorded 132 combined tackles (96 solo), seven pass deflections, three forced fumbles, three fumble recoveries, two interceptions, one sack, and a touchdown. Pro Football Focus gave Mosley an overall grade of 72.6, which ranked 20th among all qualifying linebackers in 2017. He was ranked 98th by his peers on the NFL Top 100 Players of 2018.