The current study planned to identify suspected causes of mass mortalities in Oreochromis niloticus at private fish farms in Port Said Governorate, Egypt, during the 2017 summer season. One hundred twenty diseased moribund O. niloticus fish were randomly collected from different locations. The examined fish showed lethargy, dark skin, bilateral exophthalmia with corneal opacity, and varying degrees of ulcers. Internal examination revealed generalized inflammation, congestion, hemorrhaging, and enlargement of most internal organs. Identification was accomplished through analytical profile index (API) 20E and polymerase chain reaction (PCR) and confirmed by sequencing and phylogenetic analysis. An antibiogram was constructed based on results from a disc diffusion test. Experimental infection was also performed and showed 80% mortality. Virulence genes were also assayed. Histopathological examination of experimentally challenged fish revealed prominent pathological lesions (mainly necrosis), degenerative changes, and cellular vacuolation. In conclusion, Aeromonas veronii can be considered one of the main causes of summer mass mortalities in O. niloticus in the study area.