Stem cell therapy is a potential therapeutic approach for disorders characterized by intestinal injury or loss of functional surface area. Stem cell function and proliferation are mediated by the stem cell niche. Stromal cells such as intestinal subepithelial myofibroblasts (ISEMFs) are important but poorly studied components of the stem cell niche. To examine the role of ISEMFs, we have previously generated mice with deletion of epimorphin ( Epim), an ISEMF protein and member of the syntaxin family of intracellular vesicle docking proteins that regulate cell secretion. Herein we explore the mechanisms for previous observations that Epim deletion increases gut crypt cell proliferation, crypt fission, and small bowel length in vivo. Stem cell-derived crypt culture techniques were used to explore the interaction between enteroids and myofibroblasts from Epim and WT mice. Enteroids cocultured with ISEMFS had increased growth and crypt-like budding compared with enteroids cultured without stromal support. Epim deletion in ISEMFs resulted in increased enteroid budding and surface area compared with cocultures with wild-type (WT) ISEMFs. In primary crypt cultures, Epim enteroids had significantly increased surface area and budding compared with WTs. However, stem cell assays comparing the number of Epim vs. WT colony-forming units after first passage showed no differences in the absence of ISEMF support. Epim vs. WT ISEMFs had increased Wnt4 expression, and addition of Wnt4 to WT cocultures enhanced budding. We conclude that ISEMFs play an important role in the stem cell niche. Epim regulates stem cell proliferation and differentiation via stromal contributions to the niche microenvironment. NEW & NOTEWORTHY The role of subepithelial intestinal myofibroblasts (ISEMFs) in the gut stem cell niche is controversial. We provide novel evidence supporting ISEMFs as important niche contributors. We show that the in vivo intestinal effects of deletion of myofibroblast Epim can be recapitulated in crypt stem cell cultures in vitro. ISEMFs support cocultured stem cell proliferation and enteroid growth, and these effects are augmented by deletion of Epim, a syntaxin that regulates myofibroblast cell secretion.