Abstract Introduction Turkey hosts the highest number of Syrian refugees in Europe. Refugees are often vulnerable to situational forms of psychosocial distress as a consequence of exposure to war and violence which may result in more profound mental health problems if no treatment is sought. The aim of this study is to report evidence on mental health and psychosocial support (MHPSS) needs, access and barriers to MHPSS care among Syrians refugees living in Istanbul, Turkey. Methods A cross-sectional survey was conducted among Syrian refugees aged 18 years or over in Istanbul (Sultanbeyli) between February and May 2018. Data among 1’678 Syrian refugees were collected on mental health outcomes using the Posttraumatic Stress Disorder (PTSD) Checklist (PCL-5) and the Hopkins Symptoms Checklist (HSCL-25) for depression and anxiety. We also collected data on health care utilization, barriers to seeking and continuing care as well as knowledge and attitudes towards mental health. Descriptive analyses were used. Results The prevalence of PTSD, depression and anxiety was 19.6%, 34.7% and 36.1% respectively. The treatment gap was 89% for PTSD, 90% for anxiety and 88% for depression. 249 respondents (15%) screened positive for either PTSD, depression or anxiety in our survey and self-reported emotional/behavioural problems since arriving in Sultanbeyli. Several structural and attitudinal barriers for not seeking care were reported. Conclusions To overcome the treatment gap, mental health care programmes need to be more responsive to the needs of Syrian refugees. Barriers to seeking and continuing care can be overcome by providing MHPSS services in the community which facilitate access to care. Community-based programmes of care should be supported by activities which increase awareness about mental health issues and tackle negative attitudes towards mental illness. Key messages This study provides the first nationally representative data on the mental health of Syrian refugees in Turkey, and shows that refugees have high mental health needs. Syrian refugees do not seek mental health care despite availability of mental health services in the community.