Purpose: International students with low proficiency in the English language remain at high risk of academic, social and psychological maladjustment. Using an acculturation theoretical framework, this study aims to show that acculturation stress and social support mediate the relationship between language proficiency and academic, social, and psychological adjustment among 675 international postgraduate students in Malaysian public universities. Design/methodology/approach: Correlational research design was employed to gather data from 227 females and 448 males aged between 22-45 years belonging to Asian, Middle Eastern or African countries. Out of these, 292 students were enrolled in master's degrees, and 383 students were enrolled in Ph.D. Self-administered questionnaires were used which includes Student Adjustment to the College Questionnaire (SACQ) to measure students' academic adjustment, Kwak to assess students' English language proficiency, Multidimensional Scale of Perceived Social Support (MSPSS) to assess perceived availability of social support from friends and Acculturative Stress Scale for International Students (ASSIS) to measure acculturation stress. Findings: The hypothesized model was tested using path analysis with manifest variables in Analysis of Moment Structures (AMOS) 23.0. The findings suggest that language proficiency is a significant predictor of academic, social and psychological adjustment of international students, and this relationship is partially mediated by acculturation stress and social support. Research limitations/implications: The article concludes with implications and recommendations for international student offices and program organizers to ensure conditions for successful academic, social and psychological adjustment of the international students. Originality/value: The manuscript has not been published or submitted elsewhere.