There is no consensus on the outcomes needed for the recovery and reintegration of survivors of modern slavery and human trafficking. We developed the Modern Slavery Core Outcome Set (MSCOS) to address this gap. We conducted three English-language reviews on the intervention outcomes sought or experienced by adult survivors: a qualitative systematic review (4 databases, 18 eligible papers, thematic analysis), a rapid review of quantitative intervention studies (four databases, eight eligible papers, content analysis) and a gray literature review (2 databases, 21 websites, a call for evidence, 13 eligible papers, content analysis). We further extracted outcomes from 36 pre-existing interview transcripts with survivors, and seven interviews with survivors from underrepresented groups. We narrowed down outcomes via a consensus process involving: a three-stage E-Delphi survey (191 respondents); and a final consensus workshop (46 participants). We generated 398 outcomes from our 3 reviews, and 843 outcomes from interviews. By removing conceptual and literal duplicates, we reduced this to a longlist of 72 outcomes spanning 10 different domains. The E-Delphi produced a 14-outcome shortlist for the consensus workshop, where 7 final outcomes were chosen. Final outcomes were: “long-term consistent support,” “secure and suitable housing,” “safety from any trafficker or other abuser,” “access to medical treatment,” “finding purpose in life and self-actualisation,” “access to education,” and “compassionate, trauma-informed services.” The MSCOS provides outcomes that are accepted by a wide range of stakeholders and that should be measured in intervention evaluation.