ObjectiveTo investigate the causal relationship between gastroesophageal reflux disease (GERD) and obstructive sleep apnea (OSA) with its typical symptoms (snoring and daytime sleepiness) by using Mendelian randomization (MR). MethodsThe inverse-variance weighted method was used as the main analysis method to assess the causal effect. Sensitivity and pleiotropy analyses were carried out using leave-one-out and MR-Egger analysis, and then heterogeneity tests were conducted. ResultsIn the MR analysis, genetically predicted GERD was associated with a greater risk of OSA (IVW: OR=1.528, 95%CI 1.374 to 1.699, P=5.315E?15). Additional MR results were consistent with the IVW results, and no pleiotropy or heterogeneity was found. We also discovered a significant causal relationship between GRED and snoring (IVW: OR=0.959, 95%CI 0.949 to 0.969, P=1.507E?15), and daytime sleepiness (IVW: OR=1.024, 95%CI 1.021 to 1.036, P=4.580E?5), with no evidence of pleiotropy. ConclusionThe MR study supports a causal effect between GERD and OSA with its typical symptoms (daytime sleepiness and snoring).