Washington Nationals

Fedde was considered a top prospect for the 2014 Major League Baseball draft,^[7][8] and the Nationals selected him in the first round, 18th overall.^[9] He signed with the Nationals on July 18, for $2.51 million.^[10] Fedde did not play at all in 2014 due to injury. He spent 2015 with both the Auburn Doubledays and the Hagerstown Suns, where he posted a combined 5–3 record and 3.38 ERA between the two clubs. Fedde began the 2016 season with the Potomac Nationals, and was promoted to the Harrisburg Senators in August. Fedde ended 2016 with an 8–5 record along with a 3.12 ERA.^[11]

During the Nationals' 2017 season, with the team struggling with its bullpen during the first half, Fedde was temporarily moved to a relief role. While pitching out of the bullpen, he was promoted from Class-AA Harrisburg to the Class-AAA Syracuse Chiefs on June 13, 2017.^[12] After less than a month, Fedde was switched back to starting.^[13] The move proved fortuitous for the Nationals, who lost Joe Ross from their rotation in July after he tore his ulnar collateral ligament of the elbow and saw veteran starter Stephen Strasburg lose time due to a nerve impingement in his throwing arm.

Fedde was promoted to the major leagues to start the first game of a double-header against the Colorado Rockies on July 30, 2017.^[14] On August 7, Fedde was optioned back to Syracuse after the return of Gio González from paternity leave. Fedde improved on his 9.39 ERA from the previous year in 2018, pitching to a 5.54 ERA with 46 strikeouts across 50.1 innings of work.^[15]

In the 2019 season, Fedde continued his improvement, registering a 4.50 ERA with 41 strikeouts in a career-high 78.0 innings of work.^[16] In 2020 he was 2-4 with a 4.29 ERA.^[17] He gave up a ball hit off of him with the highest exit velocity of all balls hit off of major league pitchers in the 2020 season, at 121.3 mph by Giancarlo Stanton.^[17]

On November 18, 2022, the Nationals did not tender a contract offer for the 2023 season to Fedde, making him a free agent.^[18] He had a 21-33 record and a 5.41 ERA for the Nationals.^[19]

NC Dinos

After receiving only minor league contract offers from MLB teams, Fedde signed with the NC Dinos of the KBO League on a one-year contract for $1 million, the maximum allowed for a foreign player.^[20] Fedde was named an All–Star for the Dinos in 2023.^[21] In 30 starts for the club, he registered a 20–6 record and 2.00 ERA with 209 strikeouts in 180+1⁄3 innings pitched. He won the pitching Triple Crown in turn, and became the first foreign pitcher to log 20 wins and 200+ strikeouts in a season.^[22] Following the season, he won the Choi Dong-won Award, the KBO equivalent to the Cy Young Award,^[23] and the KBO League Most Valuable Player Award.^[24]

Chicago White Sox

On December 13, 2023, Fedde signed a two-year, $15 million contract with the Chicago White Sox.^[25]