Importance: The European Society for Clinical Nutrition and Metabolism (ESPEN) and the European Association for the Study of Obesity (EASO) have recently proposed a consensus definition and diagnostic criteria for sarcopenic obesity (SO). Objective: To implement the ESPEN-EASO diagnostic algorithm to investigate the prevalence of SO and its association with outcomes in patients with solid tumor cancers, with particular regard to associations among SO, overall survival (OS), and patient quality of life (QoL). Design, Setting, and Participants: This prospective cohort study included patients diagnosed with solid tumor starting in May 7, 2013, with the last follow-up on June 30, 2022. Patients with solid tumors were categorized into SO and non-SO groups according to ESPEN-EASO criteria. The primary outcome was OS and the secondary outcomes included patient QoL and risk of intensive care unit (ICU) admission. Data were analyzed from June to December 2023. Results: A total of 6790 patients were included in the study (mean [SD] age, 59.64 [10.77] years; 3489 were female [51.4%]). The prevalence of SO was 4.36% (296 of 6790) in the whole cohort and 14.98% (296 of 1976) in the subgroup with obesity. SO prevalence increased with age. During a median (IQR) follow-up period of 6.83 (5.67-7.04) years, 2103 patients died. Cox regression analysis indicated that SO was independently associated with lower OS (hazard ratio [HR], 1.54; 95% CI, 1.23-1.92), which was observed in both men (HR, 1.51; 95% CI, 1.09-2.10) and women (HR, 1.53; 95% CI, 1.12-2.07). SO was also associated with poorer QoL and higher risk of ICU admission (odds ratio, 2.39; 95% CI, 1.06-5.29). Among the diagnostic components of SO, low hand grip strength (HGS) was the only SO component associated with poor OS (HR, 1.15; 95% CI, 1.04-1.28). Conclusions and Relevance: This cohort study of SO found that SO was significantly associated with lower OS, poorer QoL, and higher risk of ICU admission. Weak HGS, 1 of the diagnostic conditions, was the only component of SO associated with OS. The ESPEN-EASO algorithm appears to be an applicable tool to identify cancer-associated SO, which represents a major clinical complication and factor associated with risk for poor outcomes in these patients..