The Arctic region has an important role in global climate, and changes in Arctic sea ice represent one of the most visible consequences of climate change. The Greenland Sea is one of the areas of the Arctic that have undergone drastic changes. In this study, CryoSat-2 radar altimeter data were used to retrieve the distribution of sea ice freeboard in the Greenland Sea. The sea ice thicknesses converted from the freeboard were validated using the Beaufort Gyre Experiment Program upward-looking sonar data. The results showed that both the freeboard and the extent of sea ice in the Greenland Sea have remarkable seasonal variations. It was found that the freeboard and extent of the sea ice kept increasing after the Greenland Sea had entered the frozen season in October. The average sea ice freeboard reached a maximum of 0.2 m by February. Then, the sea ice began to melt, and as the sea ice extent continued to decrease, the sea ice freeboard reached a minimum of 0.13 m by September.