The success of acceptance and commitment therapy (ACT) in improving life functioning among chronic pain patients is followed by an interest in investigating mechanisms of action via which it unfolds and validating measures to assess its key constructs. The Psychological Inflexibility in Pain Scale (PIPS-II) assesses pain avoidance and fusion. This is the first study to examine the measurement models of this instrument's Greek adaptation (G-PIPS-II) in patients with different pain localizations (i.e., chronic and headache). A community heterogeneous sample of chronic pain sufferers (N = 156) and two clinical samples comprising treatment-seeking chronic pain patients (N = 149) and treatment-seeking headache patients (N = 89) were recruited from nongovernmental chronic pain support organizations and primary care centers. Exploratory and confirmatory factor analyses demonstrated an acceptable model fit of the G-PIPS-II yielding a two-factor model: avoidance (8 items) and cognitive fusion (4 items). Moderate to high correlations with theoretically related measures supported its construct validity; reliability was high for the total scale and the Avoidance subscale and medium for the Cognitive Fusion subscale. Weak measurement invariance was established across the three pain groups, suggesting that regardless of pain localization, chronic pain and headache patients understand the two latent factors in a similar way. G-PIPS-II is a psychometrically sound instrument assessing two constructs targeted for change within ACT and is deemed a conceptually meaningful scale with items having similar meanings for patients with different pain localization. (PsycINFO Database Record (c) 2019 APA, all rights reserved).