<p>In insects, photoperiod acts as a key environmental cue triggering seasonal adaptations. In <i>Aedes aegypti</i>, recent evidence from temperate Argentina shows that short-day photoperiods induce the production of diapause eggs with inhibited hatching, enhanced energy reserves, and increased survival. However, the associated morphological changes have not been characterized in detail, and it remains unknown whether they also occur under natural conditions. Here, we provide the first quantitative assessment of photoperiod-induced egg shape variation in <i>Ae. aegypti</i>, using geometric morphometrics based on elliptic Fourier descriptors. We analyzed both laboratory-reared eggs obtained under contrasting photoperiods (long vs. short) and field-collected eggs obtained weekly during the fall in Buenos Aires, Argentina. In both datasets, the main axis of morphological variation represented a transition from elongated to rounder eggs. Eggs produced under short photoperiods and those collected later in the fall were significantly rounder, in line with the seasonal decline in day length. Although photoperiod was the strongest predictor of shape change, temperature was also associated with this trait. The magnitude of morphological differences between early- and late-fall field samples was comparable to that between laboratory treatments. These findings demonstrate that photoperiod consistently influences <i>Ae. aegypti</i> egg morphology in both experimental and natural contexts and suggest that egg shape may serve as a sensitive indicator of diapause-related changes, offering a new approach to study seasonal adaptation in this vector species.</p>