Abstract Study Objective Insomnia has been linked to acute and chronic pain conditions; however, it is unclear whether such relationships are causal. Recently, a large number of genetic variants have been discovered for both insomnia and pain through genome-wide association studies (GWASs) providing a unique opportunity to examine the evidence for causal relationships through the use of the Mendelian randomization paradigm. Methods To elucidate the causality between insomnia and pain, we performed bidirectional Mendelian randomization analysis in FinnGen, where clinically diagnosed ICD-10 categories of pain had been evaluated. In addition, we used measures of self-reported insomnia symptoms. We used endpoints for pain in the FinnGen Release 5 (R5) (N = 218,379), and a non-overlapping sample for insomnia (UK Biobank (UKBB) and 23andMe, N = 1,331,010 or UKBB alone N = 453,379). We assessed the robustness of results through conventional Mendelian randomization sensitivity analyses. Results Genetic liability to insomnia symptoms increased the odds of reporting pain (odds ratio (OR) [95% confidence interval (CI)] = 1.47 [1.38–1.58], p = 4.12 × 10−28). Manifested pain had a small effect on increased risk for insomnia (OR [95% CI] = 1.04 [1.01–1.07], p < 0.05). Results were consistent in sensitivity analyses. Conclusions Our findings support a bidirectional causal relationship between insomnia and pain. These data support a further clinical investigation into the utility of insomnia treatment as a strategy for pain management and vice versa.