After attending the Penguins training camp prior to the 2014–15 season, Dumoulin was re-assigned to the WBS Penguins. As a result of numerous injuries to the Penguins lineup, Dumoulin and Bryan Rust joined the teams' lineup in December. In his tenth NHL game, Dumoulin scored his first career NHL goal against Evgeni Nabokov of the Tampa Bay Lightning, on December 15, 2014. Upon rejoining the AHL, Dumoulin led all team defensemen in assists and points with 29 and 33. Due to his play, he earned another recall to the NHL level in April 2015. He competed with the Penguins during their first-round matchup against the New York Rangers, skating an average of 14:06 minutes a night through five postseason games. On July 9, 2015, Dumoulin signed a two-year one-way contract to remain with the Penguins organization. During the 2015 offseason, Dumoulin remained in Boston with two of his former Eagles teammates to continue off-ice workouts under strength and conditioning coach Russ DeRosa. He also began skating with High End Hockey at the Breakaway Center. Once the 2015–16 season began, Dumoulin played alongside Ben Lovejoy and became the teams' most consistent defender. As such, the defensive pairing were tasked with the Penguins' top shutdown role. Dumoulin was eventually paired with Trevor Daley and continued as one of Pittsburgh's most consistent defensemen with 16 assists and a plus-11 rating by March. Once the postseason began, Dumoulin scored his first NHL playoff goal in Game 5 of the Eastern Conference Final against the Tampa Bay Lightning. His goal, which came with 0.7 seconds left in the first period, helped lead the Penguins to an overtime loss. Despite this loss, he helped the Penguins push for Game 7 and played alongside veteran Kris Letang. Dumoulin's second post-season goal came during Game 6 of the 2016 Stanley Cup Finals against the San Jose Sharks to help the team win the Stanley Cup. With a 3–1 win over the San Jose Sharks, Biddeford-born Dumoulin became the first Maine-born NHL player to hoist the Stanley Cup. Following the Stanley Cup win, Dumoulin returned to Boston and signed up for a summer course in order to finish his bachelor’s degree in marketing. Upon completing the class, he was one course away from graduating. Dumoulin returned to the Penguins lineup for the 2016–17 season with an expectation of assuming a bigger role. Dumoulin began the season tallying seven assists