BACKGROUND: This study aimed to create a comprehensive Core Outcome Set (COS) for assessing the long-term outcome (≥ 5 years) after Metabolic Bariatric Surgery (MBS), through the use of the Delphi method. METHODS: The study utilized a three-phase approach. In Phase 1, a long list of items was identified through a literature review and expert input, forming the basis for an online Delphi survey. In Phase 2, Dutch healthcare professionals involved in MBS care, defined as having at least 1 year of experience in routine follow-up or managing issues arising during follow-up, rated the importance of these items over three Delphi rounds using a 5-point Likert scale. Participants had the option to suggest additional items. Consensus was defined as 75% agreement among panelists. In Phase 3, the final COS was validated at a national conference. RESULTS: Thirty-one professionals participated in the first Delphi round. Of these, 28 (90%) completed the second round, and 24 (77%) completed the third round. The final COS, validated by 18 healthcare professionals, included various domains: short-term textbook outcome, weight loss, remission of comorbidities, quality of life, micronutrient deficiencies, lifestyle, psychopathology, long-term complications, and preoperative indication. CONCLUSIONS: The final COS offers a multidimensional approach to evaluate long-term outcomes after MBS. This COS is expected to enhance the measurement and benchmarking of MBS care, providing a more holistic view of patient outcomes.