The study examined peer relation, self-esteem and assertiveness among selected adolescents in Ilorin. It adopted an ex-post facto design since the independent variables used were not manipulated. The independent variables are peer relation and self-esteem while assertiveness is the dependent variable. A total of 251 participants were purposively selected from three different public and private schools in the Ilorin Metropolis. There were 97 males and 154 females with their age ranging from 12-21years. 217of the participants were 12-17 years of age and 341were8-21years old. 142 were Christians and 109 were Muslims. 162 attended a single school and 89 attended mixed schools. 6 participants attended boarding schools and 245 attended day schools. Index of peer relation developed by Walter et al. (1986), Index of Self-esteem of Walter (1982) and Rathus’ (1973) Assertiveness Schedule were used to assess peer relation, self-esteem and assertiveness with reliability coefficients of .94, .93 and .85 respectively. T-test for independent measures was used in testing the five hypotheses. Results showed that adolescent with higher self-esteem (M=33.85) were more assertive than those with lower self-esteem (M=24.96), [t (249) = 4.22, P<.05]. Adolescents with higher peer-relation are more assertive than those with lower level of peer-relation (M=10.60), [t (249) = 5.16, P<.05].However, gender does not affect assertiveness [t(249)=.08, P˃.05]. Also, the school attended does not determine the level of assertiveness [t = (249) = .23, P>.05]. It was concluded therefore that, peer relation and self-esteem did influence assertiveness and it is recommended that parents and authorities should be of help to refer their children or ward to psychologists when experiencing behavioural problems.