Using an index of tropical easterly wave (TEW) activity derived from spacetime‐filtered outgoing longwave radiation, we construct composites of long‐term hourly surface meteorological observations and morningtime sounding data collected near San José, Costa Rica to investigate how TEWs affect the diurnal cycle of rainfall over land. Our results indicate that TEWs enhance the frequency of occurrence of rain during convectively active conditions over the course of the diurnal cycle. By contrast, rainfall conditional intensity sensitivity to TEW phase appears more nuanced, with indications that active conditions induce a slight delay in the timing of the diurnal peak intensity but a longer duration of heavier rainfall. Analysis of associated hourly surface meteorology along with sounding profiles and derived thermodynamic parameters points to both initial vertical and time‐evolving surface conditions regulating diurnal behavior, such as greater instability and higher precipitable water in morningtime profiles under active phase conditions. Plain Language Summary Over tropical land, rainfall often follows a characteristic daily (or diurnal) cycle, with a peak in the late afternoon. This study explores how tropical easterly waves, westward‐propagating weather disturbances known to impact rainfall over many parts of the Tropics, affect the rainfall diurnal cycle at a long‐term observation site near San José, Costa Rica. Using an index of tropical easterly wave activity, and considering the diurnal cycle in terms of rainfall occurrence frequency and intensity, the results presented here indicate generally enhanced occurrence frequency over the course of the diurnal cycle during active conditions of tropical easterly waves, while intensity shows a more nuanced behavior, with active phases showing a slight lag in peak diurnal intensity with an overall lengthening of the highest intensities. Such diurnal rainfall sensitivity to tropical easterly waves is interpreted in terms of hourly surface meteorological observations and vertical profiles from morningtime soundings. Key Points Tropical easterly wave (TEW) impact on the rainfall diurnal cycle over Costa Rica is documented in surface meteorological and sounding data Convectively active phases of tropical easterly waves are associated with increased hourly rainfall frequency of occurrence Rainfall intensity sensitivity to TEW phase is more subtle, with indications of delayed active phase peak intensity