ObjectiveDiminished quality of life, inadequate support and social isolation are commonly experienced by individuals living with the chronic pain condition, endometriosis. We aimed to determine the feasibility and acceptability of EndoSMS, a psychologically-focused text message intervention designed to support individuals living with endometriosis. MethodsAs part of a two-arm parallel pilot randomised controlled trial with waitlist control, the feasibility and acceptability of a brief (3-month) version of EndoSMS was assessed using a mixed methods approach. Feasibility data (uptake, attrition, text message delivery analytics) and user acceptability (via self-report survey items and written feedback) were assessed. Qualitative data were thematically analysed using the template approach. Primary trial outcomes are not reported in this paper. ResultsFeasibility was indicated by: high conversion rate (99.1 %), low attrition (14.2 %), few opt-outs (0.02 %) and a high message delivery rate (99.8 %). Most intervention participants indicated user acceptability (mean = 4.02/5) across self-report questions. Most rated the length of the program (65.5 %), and the number (80.9 %) and language (94.5 %) of the text messages to be ‘just right’. Thematic analysis created four themes: A shared “battle”: Feeling less isolated and alone; “Be kind to yourself”: A focus on self-care, self-compassion and active coping; Keeping endometriosis at the forefront: Helpful or stressful?; Mixed perceptions surrounding the provision of general endometriosis information; and, Tailoring of text messages. ConclusionEndoSMS supportive text message program was feasible and acceptable for individuals with endometriosis. Future developments of the program should consider greater tailoring of content to user needs.Trial Registration: Australian New Zealand Clinical Trials Registry (ACTRN12621001642875).