Abstract El Niño‐Southern Oscillation (ENSO) asymmetry, characterized by stronger warm anomalies during El Niño than cold anomalies during La Niña, remains underestimated in climate models. Although nonlinear atmospheric and oceanic feedbacks are known to contribute to this asymmetry, the small‐scale processes modulating their strength remain unclear. Here, we show that interannual variations in the diurnal amplitude (DA) of sea surface temperature (SST) act to enhance ENSO asymmetry. Analyses of 35 Coupled Model Intercomparison Project Phase 6 (CMIP6) models reveal that those with larger SST DAs capture this asymmetry more realistically. In targeted model experiments, enhanced DA strengthens asymmetric zonal SST anomalies across the equatorial Pacific during ENSO events, reinforcing nonlinear air‐sea feedback. The resulting positive SSTA modification, caused by overall wind speed weakening, improves ENSO asymmetry by 38.5% in the Niño 3 region. Our finding indicates large DA as an important role in shaping ENSO nonlinearity and its simulation in climate models.