Acceptance and commitment therapy (ACT), based on the Psychological Flexibility (PF) model, has been recently applied to fibromyalgia (FM), and appeared effective in improving functioning. However, evidence for some of the processes within the PF model, self-as-context (SAC) in particular, is lacking within this population. The current study validates a measure of SAC, the Self Experiences Questionnaire (SEQ), and preliminarily investigates the role of SAC in relation to functioning in FM. Participants (N = 298, 93.3% women) self-reporting a diagnosis of FM were recruited via the Internet and completed an online survey. Measures included pain, pain acceptance and SAC, as processes, and pain interference, work and social adjustment, depression and depression-related interference, as outcomes. Confirmatory factor analysis of the SEQ suggested a bi-factor structure, with a general factor underlying all items and two sub-factors, self-as-distinction and self-as-observer (χ2 = 46.55, p = .06, comparative fit index (CFI) = .99, Tucker–Lewis Index (TLI) = .99, root mean square error of approximation (RMSEA) = .04). Component factors showed good reliability, Cronbach’s α = .90, and construct validity, supported by significant Pearson’s correlations between SEQ scores, acceptance and outcomes (r = −.14 to −.33). In multiple regression analyses, SEQ scores significantly predicted pain-related interference (β = −.17, p < .05), work and social adjustment (β = −.14, p < .05) and depression (β = −.21, p < .01), but not depression-related interference, after controlling for pain, but only significantly predicted depression after controlling pain acceptance. These preliminary results show potentially important associations between SAC and functioning in people with FM.