Background Migrants and refugees may not access mental health services due to linguistic and cultural discordance between them and health and social care professionals (HSCPs). The aim of this review is to identify the communication needs and barriers experienced by third-country nationals (TCNs), their carers, and HSCPs, as well as the strategies they use and their preferences when accessing/providing mental health services and language barriers are present. Methods We undertook a rapid systematic review of the literature (01/01/2011 – 09/03/2022) on seeking and/or providing mental health services in linguistically discordant settings. Quality appraisal was performed, data was extracted, and evidence was reviewed and synthesised qualitatively. Results 58/5,650 papers met the inclusion criteria. Both TCNs (and their carers) and HSCPs experience difficulties when seeking or providing mental health services and language barriers are present. TCNs and HSCPs prefer linguistically and culturally concordant provision of mental health services but professional interpreters are often required. However, their use is not always preferred, nor is it without problems. Conclusions Language barriers impede TCNs’ access to mental health services. Improving language support options and cultural competency in mental health services is crucial to ensure that individuals from diverse linguistic and cultural backgrounds can access and/or provide high-quality mental health services.