The 3rd century was the period from 201 to 300 A.D.

In this century, the Roman Empire saw a crisis, marking the beginning of Late Antiquity. In Persia, the Parthian Empire was succeeded by the Sassanid Empire.

In India, the Kushan Empire was replaced by the Gupta Empire. China was in the Three Kingdoms period. The Xiongnu formed the Tiefu state under Liu Qubei. Korea was ruled by the Three Kingdoms of Korea. Japan entered the Kofun period.

At about this time in Sub-Saharan Africa, the Bantu expansion reached Southern Africa.

In Pre-Columbian America, the Adena culture of the Ohio River valley declined in favor of the Hopewell culture. The Maya civilization entered its Classic Era.

After the death of Commodus in the previous century the Roman Empire was plunged into a civil war. When the dust settled, Septimius Severus emerged as emperor, establishing the Severan dynasty. Unlike previous emperors, he openly used the army to back his authority, and paid them well to do so. The regime he created is known as the Military Monarchy as a result. The system fell apart in the 230s, giving way to a fifty-year period known as the Military Anarchy or the Crisis of the Third Century, where no fewer than twenty emperors held the reins of power, most for only a few months. The majority of these men were assassinated, or killed in battle, and the empire almost collapsed under the weight of the political upheaval, as well as the growing Persian threat in the east. Under its new Sassanid rulers, Persia had grown into a rival superpower, and the Romans would have to make drastic reforms in order to better prepare their state for a confrontation. These reforms were finally realized late in the century under the reign of Diocletian, one of them being to divide the empire into an eastern and western half, and have a separate ruler for each.