Abstract Obliquity measurements for stars hosting relatively long-period giant planets with weak star-planet tidal interactions may play a key role in distinguishing between formation theories for shorter-period hot Jupiters. Few such obliquity measurements have been made to date due to the relatively small sample of known wide-orbiting, transiting Jovian-mass planets and the challenging nature of these targets, which tend to have long transit durations and orbit faint stars. We report a measurement of the Rossiter–McLaughlin effect across the transit of K2-140 b, a Jupiter-mass planet with period P = 6.57 days orbiting a V = 12.6 star. We find that K2-140 is an aligned system with projected spin–orbit angle λ = 0.5° ± 9.7°, suggesting a dynamically cool formation history. This observation builds toward a population of tidally detached giant planet spin–orbit angles that will enable a direct comparison with the distribution of close-orbiting hot-Jupiter orbital configurations, elucidating the prevalent formation mechanisms of each group.