p40, a Lactobacillus rhamnosus GG (LGG)-derived protein, transactivates epidermal growth factor receptor (EGFR) in intestinal epithelial cells, leading to amelioration of intestinal injury and inflammation. To elucidate mechanisms by which p40 regulates mucosal immunity to prevent inflammation, this study aimed to determine the effects and mechanisms of p40 on regulation of a proliferation-inducing ligand (APRIL) expression in intestinal epithelial cells for promoting immunoglobulin A (IgA) production. p40 upregulated April gene expression and protein production in mouse small intestine epithelial (MSIE) cells, which were inhibited by blocking EGFR expression and kinase activity. Enteroids from Egfrfl/fl, but not Egfrfl/fl-Vil-Cre mice with EGFR specifically deleted in intestinal epithelial cells, exhibited increased April gene expression by p40 treatment. p40-conditioned media from MSIE cells increased B-cell class switching to IgA+ cells and IgA production, which was suppressed by APRIL receptor-neutralizing antibodies. Treatment of B cells with p40 did not show any effects on IgA production. p40 treatment increased April gene expression and protein production in small intestinal epithelial cells, fecal IgA levels, IgA+B220+, IgA+CD19+, and IgA+ plasma cells in lamina propria of Egfrfl/fl, but not of Egfrfl/fl-Vil-Cre, mice. Thus p40 upregulates EGFR-dependent APRIL production in intestinal epithelial cells, which may contribute to promoting IgA production.