We report the discovery of EPIC 246851721 b, a tropical Jupiter in a 6.18-day orbit around the bright ($V=11.439$) star EPIC 246851721 (TYC 1283-739-1). We present a detailed analysis of the system using $K2$ and ground-based photometry, radial velocities, Doppler tomography and adaptive optics imaging. From our global models, we infer that the host star is a rapidly rotating ($v sin i = 74.92 $ km s$^{-1}$) F dwarf with $T_mathrm{eff}$ = 6202 K, $R_star = 1.586 R_odot$ and $M_star= 1.317 M_odot$. EPIC 246851721 b has a radius of $1.051 pm 0.044 R_J$, and a mass of 3.0$^{+1.1}_{-1.2} M_J$ . Doppler tomography reveals an aligned spin-orbit geometry, with a projected obliquity of $-1.47^{circ +0.87}_{ -0.86}$, making EPIC 246851721 the fourth hottest star to host a Jovian planet with $P > 5$ days and a known obliquity. Using quasi-periodic signatures in its light curve that appear to be spot modulations, we estimate the stars rotation period, and thereby infer the true obliquity of the system to be $3.7^{circ +3.7}_{ -1.8}$. We argue that this near-zero obliquity is likely to be primordial rather than a result of tidal damping. The host star also has a bound stellar companion, a $0.4 M_odot$ M dwarf at a projected separation of 2100 AU, but the companion is likely incapable of emplacing EPIC 246851721 b in its current orbit via high eccentricity Kozai-Lidov migration.
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