We present a calibration approach for the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) based on the reflectance properties of the instrument's onboard diffuser. This technique uses SeaWiFS as a reflectometer, measuring the reflected solar irradiance from the Earth and from the onboard diffuser. Since the sun is the common source of light for both measurements, the ratio of the SeaWiFS-measured radiances from the Earth and the diffuser provide the ratio for the reflectances of the two samples. The reflectance characterization of the onboard diffuser is the calibration reference for this approach. The result of the rationing technique is based on the linearity of the instrument's resopnse to the intensity of the input light. The calibration requires knowledge of the reflectance of the onboard diffuser at the start of the SeaWiFS mission, the response of the instrument bands, in digital numbers, for measurements of the diffuser at that time, and the change of the instrument's response over time after the start of the mission.