Organisational performance in this contemporary era is dependent on how much management is able to fully integrate the combination of employees with varied ideologies, age grades and diverse ethnic background and utilise their skills effectively. It was on this note that this study examined the effect of workplace diversity on organisational performance in selected universities in southwest Nigeria with the Social Identity Theory (SIT) and Resource-Based View Theory (RBV) underpinning the study respectively. A total of One hundred and thirtyeight (138) respondents were randomly selected from three (3) private universities in the Southwestern region namely, Achiever University, Owo, Ondo State; Afe Babalola University, Ado-Ekiti, Ekiti State and Bowen University, Iwo, Osun State . Findings from the study demonstrated that task performance was affected by age diversity with (r=0.869, p value <0.05); also, adaptive performance was influenced by gender diversity with (r=0.905, p value <0.05) and ethnic diversity had positive and significant connection with contextual performance with (r=0.959, p-value<0.05). the study concluded that achieving optimal performance via a diverse workforce which reflects the broader fabric of the society is crucial and essential for fostering a sense of belonging and ensuring that tertiary institutions is representative of the diverse perspectives and experiences of its student body. Therefore, it was recommended that management of private university should formulate policies and practices that promote equity, fair treatment, and opportunities for professional growth irrespective of age, gender and ethnicity.