A clamp and a wire harness capable of being easily installed on a fixation portion are provided. The clamp 41 includes the holder 70, which holds the outer sheath 36 accommodating an electric wire, and the fixed portion 50, which is connected to the holder 70 and fixed to the vehicle body panel P1. The fixed portion 50 includes the insertion groove 60, into which the columnar fastener B1 on the vehicle body panel P1 is inserted. The insertion groove 60 extends through the fixed portion 50 in the X-axis direction. The insertion groove 60 is shaped so as to include the mouth 61 in the plan view seen in the X-axis direction. The mouth 61 opens in the Y-axis direction in this embodiment, which is orthogonal to the X-axis direction.