Dimethyl phthalate (DMP), a widely used plasticizer, is a subchronic toxicant and endocrine-disrupting chemical that impairs reproductive health. This study determined the 96-h LC₅₀ of DMP and evaluated its sub-acute toxicity in the Mozambique tilapia Oreochromis mossambicus using a static-renewal bioassay. Fingerlings were exposed to different concentrations of DMP (0, 20, 40, 60, 80, and 100 mg/L) for 96 h. The 96-h LC₅₀ value for the fish was determined to be 42.8 mg/L. Subsequently, one-tenth of the LC₅₀ value (4.2 mg/L) was selected as the sub-lethal concentration for the sub-acute study. Fish were exposed to this sub-lethal concentration for 7 days and then allowed to recover in DMP-free water for an additional 7 days. Behavioural responses and morphological abnormalities were evaluated during both the exposure and recovery periods. DMP exposure induced significant behavioural, morphological, and physiological changes in fish, including abnormal swimming patterns, loss of equilibrium, dark pigmentation, caudal fin bending, abdominal bulging, gill lesions, and reduced liver and gonad mass. Following a 7-day recovery period in toxicant-free water, most behavioural alterations were reversed; however, persistent morphological abnormalities indicated incomplete recovery from DMP-induced toxicity. In conclusion, the present study demonstrates that DMP is toxic to O. mossambicus fingerlings, inducing significant behavioural and morphological alterations at sublethal concentrations. This study also provides the first estimate of the 96-h LC₅₀ of DMP for O. mossambicus, contributing baseline toxicity data for this ecologically and commercially important species.