Polonium was discovered in 1898 by Pierre and Marie Curie, who named it in honor to Poland, the native country of Marie Curie. It was the first element to be discovered through its radioactivity, and at the time its discovery was quite doubtful.
In the first attempt to isolate polonium, Marie Curie treated almost a ton of uraninite to obtain a small amount of a material with a radioactivity 400 times superior to that of uranium. Marckwald noticed that this material had identical chemical behavior to tellurium, suggesting the name radiotellurium.
In the same year of 1898, Marie and Pierre Curie identified another element, radium. In the following years, chemists and physicists studied of the decay tree of radium, and concluded that polonium was the last member of that decay. The atomic mass of polonium was derived from Marie Curie's calculations of the atomic mass of radium.
Polonium was undoubtfully recognized as an element in 1905, when it received its definitive place in the periodic table, in the selenium and tellurium group.