AD 84 (LXXXIV) was a leap year starting on Thursday of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Augustus and Sabinus (or, less frequently, year 837 Ab urbe condita). The denomination AD 84 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.

• Possible date of the Battle of Mons Graupius (AD 83 or 84), in which Gnaeus Julius Agricola defeats the Caledonians.^{[1][self-published source?][2]}
• Emperor Domitian recalls Agricola back to Rome, where he is rewarded with a triumph and the governorship of the Roman province of Africa, but he declines it.^{[citation needed]}
• Pliny the Younger is sevir equitum Romanorum (commander of a cavalry squadron).^{[citation needed]}
• The construction of the limes, a line of Roman fortifications from the Rhine to the Danube, has begun.^[3]
• Through his election as consul for ten years and censor for life, Domitian openly subordinates the republican aspect of the state to the monarchical.^{[citation needed]}
• Domitian increases the troops' pay by one third, thus securing their loyalty.^[4]

• Change from Jianchu to Yuanhe era of the Chinese Eastern Han Dynasty.^{[citation needed][further explanation needed]}

• Luke the Evangelist, Greek physician and martyr^{[citation needed]}
• Titus Flavius Sabinus, Roman consul married Julia Flavia (executed)^{[citation needed]}