The importance of plankton as the main food for the Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) has been widely addressed. However, the composition of the Nile tilapia diet may vary in a wide range of seasonal and spatial conditions, fish size and maturity. Therefore, this study aimed to compare the plankton patterns fed by the Nile tilapia in the southern parts of the Nile in Egypt (Lake Nasser, Aswan Reservoir and the Nile River). These areas differ in their morphological and hydrological characteristics due to the presence of the High Dam and the Aswan Low Dam. In addition, the study focused on the effect of the flood seasons on the feeding behavior of the Nile tilapia since the flood directly affects the ecosystem of the study areas and subsequently the composition of its natural food. In addition, the feeding behavior according to different sex and size was studied. A total of 454 samples of O. niloticus were collected from commercial fishing by fishermen using the common trammel gill nets. The frequency of occurrence method was utilized to assess the diet composition of O. niloticus. No significant differences were detected in stomach contents during the different flood seasons and regions for Bacillariophyceae and Cyanophyta. Nevertheless, there were significant differences in stomach contents in the different flood seasons and regions for the other planktonic food items. The results indicated that the feeding behavior of O. niloticus was not related to sex. Stomach analysis of O. niloticus in different length groups at each region revealed that some of the phytoplankton items (Bacillariophyceae and Cyanobacteria) had the same frequency, while the feeding behavior did not show a distinctive pattern for other phytoplankton items. Furthermore, there were significant differences in stomach contents for zooplankton items, with the variation of length groups. The study concluded that the composition of the Nile tilapia diet (except Bacillariophyceae and Cyanophyceae) varied according to region and season based on the previously reported food availability.