An abnormality determination device for an air-fuel ratio sensor includes a differential value calculator and an abnormality determiner. The differential value calculator is configured to calculate a differential value of an output value of the air-fuel ratio sensor which is configured to detect an air-fuel ratio of exhaust gas. The abnormality determiner is configured to determine abnormality of the air-fuel ratio sensor based on a result of comparison between a reference output value of the air-fuel ratio sensor and a predetermined threshold. The reference output value is obtained by the air-fuel ratio sensor when the differential value calculated by the differential value calculator becomes a predetermined value.