The main inscription was made of bronze letters. It says (identically on both sides):
IMP · CAES · FL · CONSTANTINO · MAXIMO · P · F · AVGUSTO · S · P · Q · R · QVOD · INSTINCTV · DIVINITATIS · MENTIS · MAGNITVDINE · CVM · EXERCITV · SVO · TAM · DE · TYRANNO · QVAM · DE · OMNI · EIVS · FACTIONE · VNO · TEMPORE · IVSTIS · REM-PVBLICAM · VLTVS · EST · ARMIS · ARCVM · TRIVMPHIS · INSIGNEM · DICAVIT
To the Emperor Caesar Flavius Constantinus, the greatest, pious, and blessed Augustus: because he, inspired by the divine, and by the greatness of his mind, has delivered the state from the tyrant and all of his followers at the same time, with his army and just force of arms, the Senate and People of Rome have dedicated this arch, decorated with triumphs.
Two short inscriptions on the inside of the central archway say:
LIBERATORI VRBIS (liberator of the city) — FUNDATORI QVIETIS (founder of peace)
Over each of the small archways, inscriptions say: