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The present cross-sectional validation study aimed to adapt and validate the Revised Cognitive-Affective Mindfulness Scale (CAMS-R) in a Spanish population of cancer patients. The sample included 425 patients (202 men, 223 women; M = 61.42, SD = 9.81) with breast, gastrointestinal, lung, and prostate cancer, recruited from multiple medical centers in Galicia, Spain. Both early- and advanced-stage patients were included. Confirmatory factor analysis (CFA) using maximum likelihood estimation was performed to test the factor structure, with model fit evaluated according to established criteria (e.g., CFI > 0.95, RMSEA < 0.06). Reliability (Cronbach's α, omega), measurement invariance, and concurrent validity were also assessed, along with regression analyses to examine predictive associations with psychological outcomes. The CAMS-R demonstrated a four-factor structure (attention, present-focused awareness, awareness, and acceptance) Confirmatory analyses showed that the four-factor model fit well (χ²/df = 2.63; CFI = 0.97; RMSEA = 0.05). The scale exhibited good internal consistency (α = 0.82-0.88). In convergent validity analyses, mindfulness was negatively associated with depression (β = -0.42, 95% CI [ - 0.55, -0.29], p < 0.001) and anxiety (β = -0.38, 95% CI [ - 0.51, -0.24], p < 0.001), and positively with well-being (β = 0.36, 95% CI [0.22, 0.49], p < 0.001). The models explained between 18% and 24% of the variance in the outcomes (R² = 0.18-0.24). Measurement invariance across gender, cancer stage, and cancer type further supported the robustness of the instrument. The Spanish version of the CAMS-R demonstrated strong psychometric properties, supporting its use as a valid and reliable tool to assess mindfulness in cancer patients in Galicia. Nevertheless, the cross-sectional design and regional sampling limit the generalizability of these results. Future research should replicate these findings in longitudinal and multi-regional Spanish-speaking populations to confirm broader applicability.