Methionine (Met) can activate mTOR to promote milk synthesis in mammary epithelial cells (MECs). However, it is largely unknown which G protein-coupled receptor (GPCR) can mediate the stimulation of Met on mTOR activation. In this study, we employed transcriptome sequencing to analyze which GPCRs were associated with the role of Met, and further used gene function study approaches to explore the role of GPR183 in Met stimulation on mTOR activation in HC11 cells. We identified 9 GPCRs including GPR183 which expression levels were upregulated by Met treatment through RNA-seq and subsequent RT-qPCR analysis. Using GPR183 knockdown and overexpression technology, we demonstrate that GPR183 is a positive regulator of milk protein and fat synthesis and proliferation of HC11 cells. Met affected GPR183 expression in a dose-dependent manner, and GPR183 mediated the stimulation of Met (0.6 mM) on milk protein and fat synthesis, cell proliferation, and mTOR phosphorylation and mRNA expression. The inhibition of PI3K blocked the phosphorylation of mTOR and AKT stimulated by GPR183 activation. In summary, through RNA-seq and gene function study, we uncover that GPR183 is a key mediator for Met to activate the PI3K-mTOR signaling and milk synthesis in mouse MECs.