A hinge assembly and a portable electronic device are provided. The portable electronic device includes a lid, a main body, and a hinge assembly. The hinge assembly pivotly connects the lid to the main body. The hinge assembly includes a pivot element, a torsion spring, and a hinge brace. The pivot element is formed on the lid and partially and rotatably received in the main body. The torsion spring is assembled between the lid and the main body. When the lid is in a closed position with respect to the main body, the torsion spring becomes constricted and accumulates potential energy. When the lid is opened from the main body, the potential energy of the torsion spring is released to drive the lid to rotate to an open position with respect to the main body. The hinge brace is attached to the main body for blocking an excessive rotation of the lid with respect to the main body.