Aim This study aimed to determine the association of biological (BAB) and psychological illness attributional beliefs (PAB) with medication adherence behaviour in patients with psychosis in Hong Kong. Methods A cross‐sectional survey of 301 outpatients with psychosis in Hong Kong was conducted from August to October 2016. The survey included a set of questionnaires examining patient medication adherence behaviours (using five behavioural items from the Medication Adherence Rating Scale), illness attributional belief, insight into the illness, and attitudes towards antipsychotics and medical professionals. The associations between these variables were analysed using path analysis and adjusted for covariates of perceived social support, experience of side‐effects, and gender. Results The data fit a model in which medication adherence behaviour was associated with illness attributional belief, insight, and attitudes (chi‐square = 32.33, p = .26; RMSEA = 0.02; SRMR = 0.04; and CFI = 0.97). BAB was positively and directly associated with medication adherence behaviour. PAB was positively and indirectly associated with medication adherence behaviour through insight into the illness and attitude towards medical professionals. PAB can strengthen the relationship between BAB and insight, and only the high PAB group exhibited a positive relationship between BAB and attitude towards medical professionals. Conclusions An integration of biological and psychological attributional beliefs in patients with psychosis is essential for better medication adherence behaviour. Future interventions should aim to modify patients' illness attributional beliefs by integrating both biological and psychological illness attribution to improve medication adherence.