Hartenstein was drafted by the Rockets in the 2nd round, 13th pick, 43rd overall in the 2017 draft. In his first two seasons in the NBA (2018-20), Hartenstein played 51 games with the Rockets, he averaged 9.5 minutes, 1.9 points, 2.7 rebounds, and 0.6 assists. On July 12, 2022, Hartenstein signed a multi-year contract with the Knicks. In two seasons with the Knicks (2022-24), Hartenstein is currently averaging 17.9 minutes, 8.3 points, 6.0 rebounds, and 1.5 assists in 130 career games. On January 3, 2024, against the Chicago Bulls, Hartenstein recorded his first career 20 rebound game tallying 20 rebounds along with 10 points, and five blocks.