Objectives Psychological models of voice‐hearing propose that auditory‐verbal hallucinations occur when inner speech is attributed to a source external to the self. Approximately half of the world's population is multilingual, and the extent to which they use a second language for inner speech depends on their experience and competency in it. Bilingualism therefore provides a natural window into the processes operating in auditory‐verbal hallucinations, but no systematic study of voice‐hearing in bilinguals has hitherto been conducted. Design A mixed‐methods observational study of psychiatric service users who hear voices and who are Welsh–English bilingual. Methods Thirty‐seven participants were interviewed about their history and use of Welsh and English and divided into three groups: those who learnt Welsh first (L1 Welsh), those who learnt English first (L1 English), and those who learnt the two languages simultaneously. Detailed phenomenological data were collected using The Mental Health Research Institute Unusual Perceptions Schedule. Results Both qualitative and quantitative data indicated very considerable variation in the extent to which voices were in Welsh, English, or both, with some voice‐hearers reporting that the predominant language of their voices had changed with time. There were modest but statistically significant associations between the predominant language of voices and age of language acquisition (late Welsh learners did not hear voices in Welsh), frequency of language use (more frequent use of Welsh was associated with more Welsh voices), and subjective language proficiency (proficiency in English was associated with a tendency to hear English voices). Conclusions Although this was a small study, it was the first of its kind. There is a need for more research on the implications of bilingualism for psychosis in particular and mental illness more generally. The results are broadly consistent with the hypothesis that hallucinated voices are misattributed inner speech. Practitioner points Assessments of people with mental health difficulties should routinely inquire whether they are multilingual and, if so, which language they prefer to use. People with mental health difficulties may have difficulty expressing complex issues and emotions in a second language, despite apparent fluency. When working with bilingual people who hear voices, mental health professionals should consider the language used by the voices when conducting assessments and proposing formulations.