Objective: To perform a systematic review and meta-analysis to estimate the effect of endometriosis on preterm birth (PB) risk. Methods: Searches were conducted in PubMed–MEDLINE, Embase, Scopus, Web of Science, Cochrane Library, Google Scholar, and SciELO for studies published in all languages from inception through April 2017. We included cohort studies evaluating pregnant women with and without endometriosis and conception either by spontaneous conception (SC) or with assisted reproductive technology (ART). Primary outcome was PB (<37 weeks), and secondary outcomes were intrauterine growth restriction (IUGR), low birthweight, small for gestational age (SGA), and birthweight. Pooled odds ratios (ORs) and its 95% confidence interval (CI) were calculated as effects, and random-effects models were used for meta-analyses. Risk of bias was assessed with the Newcastle-Ottawa Scale, and heterogeneity of effects among studies was described with the I2 statistic. Results: We identified 9 cohort studies including a total of 1 496 715 pregnancies (13 798 with endometriosis diagnosis). In women with endometriosis, the PB risk was significantly increased in both SC (OR: 1.59; 95% CI: 1.32-1.90) and ART (OR: 1.43; 95% CI: 1.14-1.79). The SGA risk was increased in women with endometriosis (OR: 1.16; 95% CI: 1.05-1.28), while the IUGR and low birthweight risks and birthweight were not affected by endometriosis. Conclusion: Endometriosis is associated with increased PB risk in both SC and women who obtained pregnancy using ART. Prospective studies evaluating relevant outcomes are needed to confirm these results.