Seasonal fast ice thickness at the island of Hopen (Barents Sea) was monitored over 40 years. Sea ice thickness variability as a climate indicator provides more quantitative information on the state of the ice cover than solely sea ice extent. Usually, starting to form just before December Hopen fast ice reaches maximum thickness in May (on average 0.99 m), before the ice starts to decay. Swell, currents, and winds interrupt the fast ice development at Hopen during several of the winters observed, leading to ice removal and new ice formation. Since 2000, no ice thicker than 1.0 m was observed. We find a trend in the ice thickness anomalies of −0.11 m per decade, coinciding with decreasing seasonal maximum ice thickness, and an increase in local surface air and water temperatures. This is consistent with the decreasing sea ice extent in the Barents Sea and the entire Arctic.