Year 120 (CXX) was a leap year starting on Sunday of the Julian calendar. At the time, it was known as the Year of the Consulship of Severus and Fulvus (or, less frequently, year 873 Ab urbe condita). The denomination 120 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• Emperor Hadrian visits Britain.
• Foss Dyke is constructed in Britain.
• A Kushan ambassadorial contingent visits with Hadrian.
• Suetonius becomes Hadrian's secretary ab epistolis.
• Approximate date
  • Legio IX Hispana last known to be in existence.
  • The Market Gate of Miletus is built at Miletos (moved in modern times to Staatliche Museen zu Berlin, Preußischer Kulturbesitz, Antikensammlung).

• Change of era name from Yuanchu (7th year) to Yongning of the Chinese Eastern Han dynasty.
• The Scythians dominate western India: Punjab, Sind, the north of Gujarat and a portion of central India.

• February 8 – Vettius Valens, Greek astrologer (d. 175)
• Irenaeus, Greek bishop and apologist (approximate date)
• Lucian, Syrian rhetorician and satirist (approximate date)
• Tatian, Syrian Christian writer and theologian (d. 180)

• Ban Zhao, Chinese historian and philosopher (b. AD 49)
• Dio Chrysostom, Greek historian (approximate date)
• Faustinus and Jovita, Roman Christian martyrs
• Getulius, Roman officer and Christian martyr
• Hermes, Greek Christian martyr and saint
• Marcian of Tortona, Roman bishop (or 117)
• Matthias of Jerusalem, bishop of Jerusalem
• Nicomachus, Greek mathematician (b. AD 60)
• Plutarch, Greek philosopher (approximate date)
• Sextus Pedius, Roman jurist (b. AD 50)
• Tacitus, Roman historian^[1]