Sea-ice extent of the Okhotsk Sea has decreased by about 20% for the past three decades, indicating that the Okhotsk Sea is sensitive to global warming. On the other hand, information of sea-ice thickness has been very limited in this sea. Its average, spatial distribution and interannual variations have not been understood at all. When sea ice melts in spring, the ocean surface is considerably freshened. Vertical integration of this salinity decrease provides estimation of the amount of sea-ice melt, giving the inference of the sea-ice thickness. In the Okhotsk Sea, 24 profiling floats, among which four floats have an ice-detection device, have been deployed since 2000. Evolution of the surface freshening observed by these floats confirms validity of the method of ice-thickness estimation. From the float observation, melting rate of sea ice can also be estimated: the rate is 1–3 cm d^–1 in thickness decrease. Spatial distribution and interannual variations of sea-ice thickness in the Okhotsk Sea are inferred from the spring salinity profiles by the hydrographic observations during the past 80 years. The averaged ice thickness estimated from all the profiles is 0.76 m. Climatological map of the ice thickness is presented for the whole Okhotsk Sea. The thickness is relatively large, typically 0.7–1.2 m, in the western and southwestern parts of the Okhotsk Sea, which is consistent with the ice-profiling sonar observations. The thickness is relatively small in the central and northeastern parts of the Okhotsk Sea. From the analysis of the interannual variations south of 53° N, where relatively a large number of hydrographic observations has been done, the averaged thickness is estimated to be 0.91 m for the 1950s–1960s, 0.83 m for 1970s–1980s and 0.66 m for the 2000s. These suggest that, as well as the ice extent, the ice thickness has also decreased in the Okhotsk Sea during the past several decades.