Background: Cachexia is common in patients with esophagogastric cancer and is associated with increased mortality. Nutritional screening and dietetic interventions can be helpful in preventing evolvement of cachexia. Our aim was to study the real-world prevalence and prognostic value of pretreatment cachexia on overall survival (OS) using patient-reported weight loss, and to explore dietetic interventions in esophagogastric cancer. Materials and Methods: Patients with esophagogastric cancer (2015–2018), regardless of disease stage, who participated in the Prospective Observational Cohort Study of Esophageal-Gastric Cancer Patients (POCOP) and completed patient-reported outcome measures were included. Data on weight loss and dietetic interventions were retrieved from questionnaires before start of treatment (baseline) and 3 months thereafter. Additional patient data were obtained from the Netherlands Cancer Registry. Cachexia was defined as self-reported >5% half-year body weight loss at baseline or >2% in patients with a body mass index (BMI) <20 kg/m2 according to the Fearon criteria. The association between cachexia and OS was analyzed using multivariable Cox proportional hazard analyses adjusted for sex, age, performance status, comorbidities, primary tumor location, disease stage, histology, and treatment strategy. Results: Of 406 included patients, 48% had pretreatment cachexia, of whom 65% were referred for dietetic consultation at baseline. The proportion of patients with cachexia was the highest among those who received palliative chemotherapy (59%) or best supportive care (67%). Cachexia was associated with decreased OS (hazard ratio, 1.52; 95% CI, 1.11–2.09). Median weight loss after 3-month follow-up was lower in patients with cachexia who were referred to a dietician at baseline compared with those who were not (0% vs 2%; P=.047). Conclusions: Nearly half of patients with esophagogastric cancer have pretreatment cachexia. Dietetic consultation at baseline was not reported in more than one-third of the patients with cachexia. Because cachexia was independently associated with decreased survival, improving nutritional screening and referral for dietetic consultation are warranted to prevent further deterioration of malnutrition and mortality.