Aphthitalite is a potassium sulfate mineral with the chemical formula: (K,Na)<sub>3</sub>Na(SO<sub>4</sub>)<sub>2</sub>.
It was first described in 1835 for an occurrence on Mount Vesuvius, Italy. The name is from the Greek άφθητος, "unalterable", and άλας, "salt", for its stability in air.