The specific role of the chloride anion (Cl−) as a signalling effector or second messenger has been increasingly recognized in recent years. It could represent a key factor in the regulation of cellular homeostasis. Changes in intracellular Cl− concentration affect diverse cellular functions such as gene and protein expression and activities, post‐translational modifications of proteins, cellular volume, cell cycle, cell proliferation and differentiation, membrane potential, reactive oxygen species levels, and intracellular/extracellular pH. Cl− also modulates functions in different organelles, including endosomes, phagosomes, lysosomes, endoplasmic reticulum, and mitochondria. A better knowledge of Cl− signalling could help in understanding the molecular and metabolic changes seen in pathologies with altered Cl− transport or under physiological conditions. Here we review relevant evidence supporting the role of Cl− as a signalling effector.