On September 16, 2014, the San Francisco 49ers signed Grimble to their practice squad. On November 18, 2014, he was waived by the 49ers. On December 16, 2014, he was re-signed to the 49ers practice squad. On December 31, 2014, the 49ers signed Grimble to a two-year, $960,000 reserve/future contract  On November 26, 2014, the New England Patriots signed Grimble to their practice squad, but was released two days later. On December 16, 2014, Grimble was signed to the 49ers' practice squad. He signed a reserve/future contract with the 49ers on December 30, 2014. On September 5, 2015, Grimble was waived by the 49ers. On January 18, 2016, the Pittsburgh Steelers signed Grimble to a one-year, $450,000 future/reserve contract. He entered the Steelers training camp competing for the backup tight end position with David Johnson and Paul Lang. Grimble was named the Steelers' third tight end on their depth chart to begin the season, behind veterans Jesse James and David Johnson. He made his professional regular season debut in the Steelers' season-opening 38-16 victory over the Washington Redskins. The following game, he made his first career catch and caught his first career touchdown reception on a 20-yard pass from Ben Roethlisberger in the 24–16 victory over the Cincinnati Bengals. In Week 3, he earned his first official career start during a 3-34 loss to the Philadelphia Eagles. On December 25, 2016, Grimble caught his first career touchdown on a twenty-yard pass from quarterback Ben Roethlisberger on the opening drive of a 31-27 last-minute comeback victory over the Baltimore Ravens. On February 8, 2017, Grimble signed a one-year deal to remain with the Steelers. Throughout training camp, Grimble competed with David Johnson and rookie Jake McGee for the backup tight end position. Although he showed more capability than both players, the Steelers traded for his former 49ers teammate Vance McDonald, making him the third tight end on their depth chart to begin the regular season. In the 2017 season, he appeared in 15 games and recorded five receptions for 32 receiving yards and one receiving touchdown. In the 2018 season, Grimble appeared in all 16 games, of which he started two. He recorded six receptions for 86 receiving yards. On March 5, 2019, the Steelers placed an original round tender on Grimble. He was placed on injured reserve on September 25, 2019, with a calf injury.