Background/Objectives: Refeeding syndrome (RFS) is a potentially life-threatening complication of nutritional support in critically ill patients. Its incidence and clinical relevance in patients requiring extracorporeal membrane oxygenation (ECMO) are unknown. We retrospectively investigated the incidence, characteristics and clinical relevance of RFS in COVID-19 patients receiving venovenous (vv-)ECMO support. Methods: In a secondary analysis of 101 COVID-19 patients treated with vv-ECMO at a tertiary center, RFS was defined by a decrease of more than 0.16 mmol/L in serum phosphate to a value below 0.65 mmol/L within seven days of starting medical nutrition therapy (MNT), or a pre-existing phosphate below 0.65 mmol/L. Electrolyte trajectories, nutritional delivery, predictors of RFS, and the association with ICU mortality were analyzed. Robustness was assessed across alternative published definitions. Results: RFS developed in 46 of 101 patients (45.5%), with an incidence ranging from 23.8% to 84.2% depending on the definition applied. Patients with RFS had a lower body mass index and required significantly more phosphate substitution than those without (93.5% vs. 43.6%; p < 0.001). Early energy delivery did not differ between groups and was not associated with RFS. RFS was not associated with ICU mortality (41.3% vs. 41.8%; adjusted hazard ratio 0.80, p = 0.51), a finding consistent across all definitions examined. Conclusions: In this COVID-19 vv-ECMO cohort, refeeding-associated hypophosphatemia was frequent but not associated with caloric delivery or ICU mortality under close electrolyte monitoring. These hypothesis-generating findings do not support reducing caloric intake for isolated hypophosphatemia and require prospective confirmation.