亚历山大一世岛（Alexander I Island），亦称亚历山大岛（Alexander Island），南极洲沿岸一岛屿，位于南极半岛以西的别林斯高晋海中，面积43250平方公里。

俄罗斯帝国探险家别林斯高晋于1821年1月28日发现该岛，并以俄罗斯帝国皇帝亚历山大一世之名为其命名。1940年之前，人们均认为该岛是南极洲大陆的一部分，此后才发现该岛与南极大陆并不相连。

英国、阿根廷和智利对该岛有主权要求，但在南极条约体系中被冻结。

Alexander Island, which is also known as Alexander I Island, Alexander I Land, Alexander Land, Alexander I Archipelago, and Zemlja Alexandra I, is the largest island of Antarctica. It lies in the Bellingshausen Sea west of Palmer Land, Antarctic Peninsula from which it is separated by Marguerite Bay and George VI Sound. George VI Ice Shelf entirely fills George VI Sound and connects Alexander island to Palmer Land. The island partly surrounds Wilkins Sound, which lies to its west. Alexander Island is about 390 kilometres (240 mi) long in a north-south direction, 80 kilometres (50 mi) wide in the north, and 240 kilometres (150 mi) wide in the south. Alexander Island is the second largest uninhabited island in the world, after Devon Island.