The freshwater macroalga Aegagropila linnaei can occur attached or unattached, but is best known for its free-floating spherical growth form (“lake balls,” “Cladophora balls,” or “marimo””). The worldwide distribution of this species was reconstructed using more than 1200 specimens from 28 herbaria, an extensive literature survey, and field observations. Aegagropila linnaei was found in 283 locations, with the majority located in central and northern Europe. Few records of A. linnaei exist from North America, but it is relatively widespread in Japan. The preferred habitat is inferred to be oligomesotrophic lakes with moderate to high calcium levels. In more than 50% of the known locations, A. linnaei has not been found in the last 30 years, and this decline is assumed to be correlated with changes in trophic conditions in those lakes. The unattached growth forms show a stronger decline than attached populations as a result of the habitat types in which they can occur.