Thallium (Tl) is a chemical element with atomic number 81. There are two stable thallium isotopes, ²⁰³Tl and ²⁰⁵Tl, of which ²⁰⁵Tl is the more abundant.

Thallium isn't found in pure form in nature. It is a very toxic element, and chemical compounds such as thallium sulfate (Tl₂SO₄) was previously used as rat poison.