Background: Mental illnesses are health conditions which are associated with changes in emotion, thinking, or behaviour (or a combination of these). Healthcare-seeking behaviour for formal mental health treatment is lacking all over the world, particularly in low and middle-income countries. Inappropriate health-seeking behaviours are reported to result in delays in seeking appropriate care and thus increase the risk of complications in mentally ill patients. The study aimed to assess factors influencing formal mental treatment-seeking behaviour among caretakers of mentally ill patients in Zanzibar Methods: A community-based cross-sectional study design was conducted from January to June, 2021. A total of 246 caretakers of mentally ill patients were recruited for the study using multi-stage sampling technique. An interviewer-administered semi-structured questionnaire was used to collect information from caretakers. Bivariate and multivariable logistic regression models were applied to determine the factors influencing formal mental treatment-seeking behaviour. Results: Majority of caretakers 187(76%) were aware of formal mental treatment. Also, majority of the participants 145(58.9%) had appropriate healthcare-seeking behaviour toward formal mental treatment. Factors influencing formal mental treatment-seeking behaviour were; perceived severity (AOR 4.651 at 95% CI 2,397-9.021 p<.001) and being aware (AOR 2.907at 95% CI 2.349-2.326 p=.004). Conclusion: Majority of caretakers were aware of formal mental illness treatment. Also,more than half of the caretakers had appropriate healthcare-seeking behaviour. Factors associated with formal mental treatment-seeking behaviour were awareness of formal mental treatment and perceived severity of mental illness. The study recommends a community sensitisation campaign to raise community awareness and perception towards formal mental treatment. Community sensitisation is crucial for improving formal mental treatment-seeking behaviour.