SIXTUS SIXTUS, the name of not less than five popes. S. I., St., Pope from 116 to 125; S. II., st., pope from 257 to 259; S. III., Pope from 432 to 440; S. IV., pope from 1471 to 1484; S. V., Pope from 1585 to 1590; of whom only two are of any note.