Protactinium, formerly protoactinium, (symbol Pa), is a radioactive metallic element with an atomic number of 91.

Protactinium is a member of the actinide series of the periodic table.

It was discovered in 1918 by the Austrian-Swedish physicist Lise Meitner and the German physical chemist Otto Hahn.

Protactinium is a member of the uranium-actinium radioactive-decay series and is found in uranium ores.
Isotopes of protactinium ranging in mass number from 215 to 238 are known. Protactinium-233 has a half-life of 27 days. Protactinium-231, the most stable isotope, has a half-life of more than 32,000 years; by emission of an alpha particle it decays to actinium.

Protactinium melts at about 1552° C (about 2826° F), boils at about 4227° C (about 7641° F), and has a specific gravity of about 15.37.

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