Abstract: This study investigates the effects of a moderate geomagnetic storm during July 13-14, 2013 on the development of ionospheric irregularities in the low latitude region of China. Although in the East Asian sector, ionospheric irregularities are relatively uncommon in July, unseasonal irregularities are observed during this geomagnetic storm. Ground-based GNSS data from Crustal Movement Observation Network of China (CMONOC) and Hong Kong Satellite positioning Reference stations (SatRef), S4 index data from the ionospheric scintillation monitor (ISM) at Shenzhen (22.59°N, 113.97°E) station, ionosonde data from Fuke (19.4°N, 109.0°E) station, and geomagnetic data are used in this study to show the evolution of the ionospheric irregularities in the low latitude region of China during the geomagnetic storm. Geomagnetic and solar activity parameters are used to analyze the generation mechanisms of the irregularities. It is shown that the eastward electric field enhanced by the geomagnetic storm is the major factor contributing to the generation of the irregularities. Satellite traces in the ionograms before the onset of the irregularities indicate that large-scale wave structures acting as seed perturbations may also contribute to this event. Furthermore, it is found that the temporal distribution of S4 index and ROTI are generally similar (enhanced during 15:00-18:00 UT) and coincide with the time range of the irregularities, but the distribution of L2 loss of lock is slightly different (occurring around 14:00 UT, about one hour earlier). These loss of lock occurrences are unrelated to this event.