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Nonalcoholic fatty liver disease (NAFLD), now largely encompassed by the newer metabolic dysfunction-associated steatotic liver disease (MASLD) nomenclature, is increasingly recognized as a marker of systemic metabolic dysfunction. This systematic review synthesized longitudinal evidence from adults examining NAFLD and related steatotic liver disease definitions in relation to incident type 2 diabetes mellitus. PubMed/MEDLINE and Scopus were searched from inception, the supplementary search was repeated to capture recently published studies. Eligible longitudinal reports excluded participants with diabetes at baseline or clearly identified new-onset diabetes during follow-up. Because the included cohorts differed in liver disease definitions, diabetes ascertainment, populations, follow-up structures, and reported effect measures, the findings were synthesized narratively. The protocol was prepared before screening but was not prospectively registered. A total of 61 reports were included. Risk of bias was generally low to moderate where adequately assessed, while reports with incomplete domain-level information were interpreted cautiously. NAFLD or MASLD was generally associated with higher estimates of incident diabetes risk, although the magnitude varied and was attenuated after adjustment for metabolic factors in some studies. Persistent or newly developed steatosis and a greater liver fat or fibrosis burden were associated with higher estimates of future diabetes occurrence, whereas the resolution or reduction of liver fat was associated with lower estimates. Reported subgroup differences arose largely from individual or overlapping cohorts and were not sufficiently consistent to establish uniform effect modification. Overall, the evidence supports NAFLD or MASLD as a clinical risk marker rather than a stand-alone individualized prediction tool and supports standardized longitudinal assessments in future studies.