Secondary School students (SSs) are important members of the community; hence their attitude towards mental illness can be highly influential. Mentally ill individuals are not only suffering from the illness but also suffering from the stigmatizing attitude generated by the community. The objectives of this study were to determine attitude of SSs towards mental illness and its associated factors. A cross-sectional study design employing stratified random sampling was applied to select a sample of 402 students. Data was obtained using a self-administered Belief towards Mental Illness (BMI) questionnaire. Independent sample t-tests and one-way ANOVA were used to determine possible differences in scores of attitude. From a total of 21 BMI scale items, positive attitudes were found in eight items and negative attitudes were found in the remaining thirteen. The mean score of the full BMI scale was 2.47 (95% CI: 2.41, 2.54). The mean (95% CI) scores of dangerousness, poor social relations and incurability, and shame subscales were 2.68 (2.60, 2.76), 2.55 (2.48, 2.62), and 1.22 (1.09, 1.34), respectively. A significant negative correlation was found between attitude scores and the average mark of students (r = -0.257, p<0.0001). Moreover, significant differences in attitude scores were observed between students with a relative of mental illness and those without such a relative (p=0.004). There was an increasing trend of positive attitudes with increased educational level among 9th, 10th, and 11th graders (p-trend<0.0001) and with an increase in the educational level of the students father (p-trend=0.028). However, no significant difference in attitude score was found across categories of sex, religion, living condition of father, presence of a mentally ill neighbor, educational level of mother, or ethnicity. In conclusion, considerable numbers of SSs have negative attitudes towards mental illness. Implementation of programs that enhance positive attitudes towards mentally ill individuals is recommended.