We present a detailed analysis of HARPS-N radial velocity observations of K2-100, a young and active star in the Praesepe cluster, which hosts a transiting planet with a period of 1.7 d. We model the activity-induced radial velocity variations of the host star with a multidimensional Gaussian Process framework and detect a planetary signal of 10.6 ± 3.0 |${\rm m\, s^{-1}}$|⁠, which matches the transit ephemeris, and translates to a planet mass of 21.8 ± 6.2 M. We perform a suite of validation tests to confirm that our detected signal is genuine. This is the first mass measurement for a transiting planet in a young open cluster. The relatively low density of the planet, |$2.04 _{ - 0.61 } ^ { + 0.66 }$| |${\rm g\, cm^{-3}}$|⁠, implies that K2-100b retains a significant volatile envelope. We estimate that the planet is losing its atmosphere at a rate of |$10^{11}\!-\!10^{12}\, {\rm g\, s^{-1}}$| due to the high level of radiation it receives from its host star.

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