Recent insights supporting the fallopian tube epithelium (FTE) and serous tubal intraepithelial carcinomas (STIC) as the tissue of origin and the precursor lesion, respectively, for the majority of high-grade serous ovarian carcinomas (HGSOC) provide the necessary context to study the mechanisms that drive the development and progression of HGSOC. Here we investigate the role of the E3 ubiquitin ligase RNF20 and histone H2B monoubiquitylation (H2Bub1) in serous tumorigenesis and report that heterozygous loss of RNF20 defines the majority of HGSOC tumors. At the protein level, H2Bub1 was lost or downregulated in a large proportion of STIC and invasive HGSOC tumors, implicating RNF20/H2Bub1 loss as an early event in the development of serous ovarian carcinoma. Knockdown of RNF20, with concomitant loss of H2Bub1, was sufficient to enhance cell migration and clonogenic growth of FTE cells. To investigate the mechanisms underlying these effects, we performed ATAC-seq and RNA-seq in RNF20 knockdown FTE cell lines. Loss of RNF20 and H2Bub1 was associated with a more open chromatin conformation leading to upregulation of immune signaling pathways, including interleukin 6 (IL6). IL6 was one of the key cytokines significantly upregulated in RNF20- and H2Bub1-depleted FTE cells and imparted upon these cells an enhanced migratory phenotype. These studies provide mechanistic insight into the observed oncogenic phenotypes triggered by the early loss of H2Bub1.