A unique and novel organic compound has been detected in sea ice samples from three locations in the Canadian Arctic, thousands of kilometers apart. It is likely that this biomarker is produced by diatoms living in the sea ice and we provide evidence which suggests that the compound, a C25 monounsaturated hydrocarbon (IP25), may be a specific, sensitive and stable proxy for sea ice in sediments over at least the Holocene. Since it has not been reported before, we confirmed its identity by synthesis and used the synthesized compound as a reference for quantifying IP25 in a range of sediments from an East-West transect in the Canadian Arctic. The recent sediments are either seasonally covered with ice or have permanent ice cover. Older sediments containing IP25 have been dated using radiocarbon methods to at least 9000 yr. If the concept proves generally applicable, monitoring IP25 in further sediment cores along with other accepted proxies, should allow movements in the position of the ice edge throughout the Holocene (at least) to be better determined, which is essential for accurate calibration of climate prediction models. A similar approach for Antarctic samples of ice and sediments would also seem worthwhile.