Rationalis was the Roman Empire's chief financial minister prior to the reforms of Emperor Diocletian and the Late Empire. Among the tasks of the rationalis were the collection of all normal taxes and duties, the control of currency, and the administration of mines and mints. After the reforms of Diocletian, the rationalis and the Rationibus were replaced by the comes sacrarum largitionum.

See also Aerarium, comes, Congiarium, fiscus, rationibus, and Roman finance.