Rubidium (Rb) is a metallic chemical element with atomic number 37. The most abundant isotope of rubidium is ⁸⁵Rb, which has 37 protons and 48 neutrons.

Rubidium is part of a group of elements called alkali metals, together with lithium, sodium, potassium, caesium and francium. Like other alkali metals, rubidium is highly reactive.

While time units are officially defined from the caesium atom, rubidium can also being used in atomic clocks for extremely accurate timing.