Abstract The mTOR complex 1 (mTORC1) coordinates several important environmental and intracellular cues to control a variety of biological processes, such as cell growth, survival, autophagy, and metabolism in response to energy levels, growth signals, and nutrients. The endoplasmic reticulum (ER) is a crucial intracellular organelle that is essential for numerous cellular functions, including the synthesis, folding and modification of newly synthesized proteins, stress responsiveness, and maintainence of cellular homeostasis. mTOR-mediated upregulation of protein synthesis induces the accumulation of misfolded or unfolded proteins in the ER lumen which induces ER stress, leading to activation of the unfolded protein response (UPR) pathway. Reciprocally, ER stress regulates the PI3K/AKT/mTOR signaling pathway. Therefore, under pathological conditions, the crosstalk between the mTOR and UPR signaling pathways during cellular stress can critically affect cancer cell fate and may be involved in the pathogenesis and therapeutic outcome of cancer. Here, we discuss accumulating evidence showing the mechanism of action, interconnections, and molecular links between mTOR signaling and ER stress in tumorigenesis and highlight potential therapeutic implications for numerous cancers.