We announce the discovery of EPIC218916923 b, a transiting warm-Jupiter (Teq=555±11 K) on a 29-day orbit around an active (logR′HK = −4.49 ± 0.03) K0 V star in K2 Campaign 7. We derive the system's parameters by combining the K2 photometry with ground-based follow-up observations. With a mass of Mp=0.381±0.045MJup and radius of Rp=0.812±0.030RJup, EPIC218916923 b is one of the transiting warm Jupiters with the lowest mass known to date. The planetary mean density ρp=0.88±0.14gcm−3 can be explained with a core of ∼50M⊕. Given the brightness of the host star (V= 11.653 mag), the relatively short transit duration (∼5 hours), and the expected amplitude of the Rossiter-McLaughlin effect (∼25 ms), EPIC218916923 is an ideal target to measure the spin-orbit angle of a planetary system hosting a warm Jupiter.