Actinium was discovered by Debierne in 1899 and independently by Gisel in 1902, in the rare earth fraction of pitchblende residues. The name actinium (aktis, ray) was proposed by Debierne.

The genetic relationships of the members of the actinium decay series were determined during the next twenty years by radiochemical studies. The parent of the series, U235, and the mass numbers of all its members were established by Dempster in 1935, by mass spectrometric analysis.