Diocletian's Tetrarchy reforms of 296 alter Cyrenaica's administrative structure.

It is split into two provinces: Libya Superior or Libya Pentapolis, comprising the above-mentioned Pentapolis, with Cyrene as capital, and Libya Inferior or Libya sicca, the Marmarica, which has by this time gained a significant city, the port Paraetonium.

Each comes under a governor holding the modest rank of praeses.

Both belong to the Diocese of the Orient, with its capital at Antioch in Syria, and from 370, to the Diocese of Egypt, within the praetorian prefecture of Oriens.

Its western neighbor Tripolitania, the largest split-off from Africa proconsularis, becomes part of the Diocese of Africa, subordinate to the prefecture of Italia et Africa.

Following the Crete earthquake of 365, the capital is moved to Ptolemais.

After the Empire's division, Cyrenaica becomes part of the East Roman Empire (Byzantine Empire), bordering Tripolitania.

It is briefly part of the Vandal Kingdom to the west, until its reconquest by Belisarius in 533.