How mesenchymal stem cells (MSCs) promote tumor growth remains incompletely understood. Here, we show that mesenchymal stem-like cells (MSLCs) are commonly present in malignant pleural effusion or ascites of cancer patients, where they directly interact with tumor cells. Chemokines and chemokine receptors, especially the CCL2/CCR2 pathway, are involved in this interaction. As a result, MSLCs exert tumor-promoting effects by enhancing the proliferation and colony formation of tumor-repopulating cells. The underlying molecular basis involves MSLC release of glutamine to tumorigenic cells. Inhibition of glutamine uptake impedes MSC-mediated tumor-promoting effects. More intriguingly, MSLCs take up tumor cell-released ammonium that, in turn, favors MSLC growth. Thus, glutamine and ammonium form a vicious cycle between MSLCs and tumorigenic cells. These findings suggest a potential clinical application by targeting MSLCs in patients with malignant pleural effusions or ascites.