Urban warming due to increased urbanization is becoming a serious environmental problem, requiring urban planners to consider heat mitigation strategies that reduce urban air temperature. Urban rivers play an important role in reducing urban heat through evaporation and transfer of sensible heat, known as the river cooling effect (RCE). We used detailed field measurements to calculate the river cooling intensity (RCI) and river cooling distance (RCD) for the Cheonggye River in Seoul, Korea in order to determine the relationship between RCE and urban form at different times of day during summer. Our results showed that the Cheonggye River had a mean RCI of 0.46 °C and a mean RCD of 32.7 m at 2 p.m. and a mean RCI of 0.37 °C and a mean RCD of 37.2 m at 10 p.m. Spatial variations in RCE were negatively correlated with street width and mean building height at 2 p.m., indicating that narrower streets and lower buildings would improve the RCE. In addition, temporal variations in RCE were related to changes wind speed at similar humidity levels. Our results show that the urban form surrounding a river can affect the local RCE, suggesting that landscape and urban planners should consider urban form as a variable affecting urban heat and RCE.