Siganus rivulatus is one of the most successful Lessipsian migrant fish from the Red Sea to the Mediterranean Sea through Suez Canal. In the present study, a comparison between the populations in native and new habitats was estimated based on morphometric characters, meristic count, and condition of the fish. A total of 1741 individuals of S. rivulatus (334 from the Red Sea; Hurghada, 581 from Suez Canal; Ismailia and 826 from the Mediterranean; Port-Said) were collected seasonally from autumn 2017 to summer 2018. The total length (TL) fluctuated between 14-28cm in the Red Sea with the mode at length-class 16-16.9cm (14.37%), while TL in Suez Canal and the Mediterranean ranged from 8 to 22cm, the mode was 10-10.9cm (25.3%) and 12-12.9cm (15.25%), respectively. TL, SL and TW displayed significant differences between sites (ANOVA, P≥0.05), with the higher values of all morphometric parameters for Red Sea population. The meristic equation was “D, XIV+10; A, VII+9; V, I+3+I; P, 15-16" at all studied sites. Length-weight relationships were estimated as W=0.0112L^3.0349, W=0.0085L^3.1825, and W=0.0154L^2.9091 for the Red Sea, Suez Canal and the Mediterranean, respectively. Seasonal variation in Fulton's condition factor (Kf) and Clark's (Kc) were estimated based on the total weight (TW) and gutted weight (GW) respectively. The values of Kf and Kc showed significant differences between sites (ANOVA, P≥0.05). Kf seemed to be affected by temperature as the maximum value recorded in summer while the minimum was in winter. was considered another factor that may affect the condition of the fish as spring (reproductive season) showed high decrease in the value of Kc at all sites when we removed the mature gonad.