Actinium (Ac) is a radioactive chemical element with atomic number 89. It is the first element in the element series called actinides which stretches from actinium to lawrencium, and is found at the very bottom of the periodic table. These fifteen elements are all fairly similar.

The name actinium comes from the Greek word aktinos, meaning "ray". Being very rare and radioactive, there are no major industrial use cases for actinium.