Context: Antibodies to the 65 kD isoform of glutamic acid decarboxylase (GAD65) have been associated with incident Type 2 Diabetes Mellitus, however results are inconsistent. Objective: To assess the association between GAD65 antibody positivity and incident Type 2 Diabetes Mellitus in a non-diabetic adult (≥18 years) population, in a systematic review and meta-analysis. Data Sources: A systematic literature search was conducted in Pubmed (MEDLINE) and Embase until January 14th, 2019. Study Selection: Included studies were 1) prospective studies on the association between GAD65 antibodies and incident Type 2 Diabetes Mellitus; 2) in a non-diabetic adult (≥18 years) population. To strengthen the review, unpublished data from 1302 Hoorn Study participants were included. Data Extraction: Data extraction and quality assessment were performed independently by two observers. Ten studies were rated for methodological quality and seven were pooled using a random-effects meta-analysis, of which 2 strong, 2 moderate and 3 of low methodological quality. Data Synthesis: The pooled risk estimate of incident Type 2 Diabetes Mellitus for GAD65 antibody positivity, compared to GAD65 antibody negativity was 3.36 (95% CI: 1.9–5.9). This result was robust to sensitivity analyses. Heterogeneity between studies was significant with I 2 statistic of 79% (p < 0.0001). However, excluding one study showed a decrease of I 2 to 19% (p < 0.0001), explaining a large part of the heterogeneity. Conclusion: GAD65 antibody positivity was associated with an increased risk of future Type 2 Diabetes Mellitus in adults.