Objectives: This study aimed to examine the psychometric properties of the Sussex-Oxford Compassion for Others Scale (SOCS–O) in a large sample of Spanish undergraduate nursing students. Method: After a forward–backward translation process, we conducted a cross-sectional study among nursing students in their first year of training at two Spanish universities. The mean age of the participants (n = 683) was 22.74 years old, and 83.46% were women. Together with compassion for others, mindfulness was also assessed. Results: Descriptive results revealed high scores across all dimensions of the SOCS–O. The data supported a 3-factor structure with correlated factors (χ ²(167) = 363.47, p < 0.01; CFI = 0.96; TLI = 0.95; RMSEA = 0.05, 90% CI = 0.04, 0.05; SRMR = 0.04). Reliability estimates, measured by Cronbach’s alpha, were excellent, ranging from 0.87 to 0.93. Measurement invariance across gender was confirmed. Men exhibited significantly lower levels in all compassion dimensions: Recognizing suffering (mean difference = − 0.40), Understanding the universality of suffering (mean difference = − 0.33), and Behavioral compassion (mean difference = − 0.57) (all p < 0.01). The structural equation model showed a clear relationship between mindfulness and compassion (χ ²(581) = 1345.79, p < 0.01; CFI = 0.92; TLI = 0.92; RMSEA = 0.04, 90% CI = 0.04, 0.04; SRMR = 0.06). Conclusions: The findings showed that the Spanish version of the SOCS–O has robust psychometric properties. The SOCS–O is a reliable tool for measuring compassion with three subscales and helps advance understanding of compassion among student nurses. The positive relationship between mindfulness and compassion suggests that incorporating mindfulness training into nursing curricula could enhance the delivery of compassionate care. Preregistration: This study was not preregistered.