The results from the study on concurrent measurements of sea-ice thickness from a ship and passive microwave radiometry from an over-flying aircraft over the Sea of Okhotsk indicate that both the brightness temperature and emissivity increase with thickness within the thin-ice category. This relationship was observed for a frequency range of 10–37 GHz. The relationship was more pronounced at lower frequencies and at the horizontal polarization. In this study, on the basis of this relationship we estimate sea-ice thickness (different ice categories) for the Sea of Okhotsk. For the estimation we used brightness temperatures of 18 GHz H-pol and V-pol from AMSR-E, and sea-ice concentration using bootstrap algorithm. R18, the ratio of 18GHz V-pol to 18GHz H-pol was computed and compared with grand-truth data taken from the ship in the Sea of Okhotsk. This comparison provided a basis for a classification scheme for three ice-thickness categories representing sea ice of approximately <10 cm, 10–20 cm and >20 cm thickness. Applying this classification method, daily maps of sea-ice thickness for seven winter seasons (2002–2008) were constructed for the Sea of Okhotsk. The results suggest that the area fraction of the thinnest ice category is about 20–40%, which tends to increase during the early winter period. Spatially the thin area covers the coastal region, in which polynyas are known to exist, and the areas along the ice edge. The time series of both the thin-ice fraction and SLP show covariability with jumps, which point to strong influences on sea-ice variability from the passage of low-pressure systems over the region. Overall, the method appears to capture characteristics of both spatial and temporal variations in the thin-ice region within the SIZ.