Significance The epithelium barrier is vital for sealing body surface and alimentary spaces, preventing paracellular material diffusion and pathogen invasion. We describe here that tuberous sclerosis complex 1 (Tsc1) controls tight junction (TJ) formation to create and maintain the epithelial barrier, independent of its conventional role in mTOR regulation. We found that Tsc1 loss is associated with TJ destruction in patients with TJ-related human diseases, such as inflammatory bowel disease and psoriasis, a common skin disorder. Tsc1 knockout in epithelial cells drives TJ dysfunction, causing Crohn’s disease–like and psoriasis-like signs and symptoms in mice. This function of Tsc1 has significant implications to understand how epithelial cells form TJ to protect us from these diseases.