Purpose <p>The causal relationship between childhood adiposity and gynecologic cancers remains unclear. We performed a two-sample Mendelian randomization (MR) study to elucidate the association between childhood adiposity and the risk of gynecologic cancers.</p> Methods <p>The three distinct indicators of childhood adiposity that constitute the exposures were childhood body mass index (CBMI), childhood body size at age 10 (CBS-10) and childhood obesity (COBE). In tandem, the study scrutinized the outcomes encompassing gynecologic cancers, including ovarian cancer (OC), endometrial cancer (EC), cervical cancer (CC) and their subtypes.</p> Results <p>The results of the inverse variance weighted (IVW) method suggested that CBMI was positively associated with OC (OR = 1.219, 95% CI, 1.084–1.370, q-value = 9.45E-04), EC (OR = 1.417, 95% CI, 1.272–1.702, q-value = 2.04E-07) and some of their subgroups. There were positive association between CBS-10 and invasive mucinous ovarian cancer (IMOC) (OR = 1.923, 95% CI, 1.184–3.125, q-value = 0.008), EC (OR = 1.727, 95% CI, 1.396–2.137, q-value = 4.80E-07) and its subtypes as indicated by IVW. And it is suggested by IVW that COBE was positively associated with EC (OR = 1.088, 95% CI, 1.019–1.163, q-value = 0.012). Additionally, there was no association between CBMI, CBS-10 and COBE and the risk of CC.</p> Conclusions <p>Overall, this study indicates that childhood adiposity is causally associated with ovarian and endometrial cancers at the genetic level, but childhood adiposity is not causally associated with cervical cancer.</p>