Arathorn I is a fictional character from J. R. R. Tolkien's universe of Middle-earth.

Arathorn I was the twelfth of the Chieftains of the Dúnedain.

Like his forefathers, he was raised by Elrond in Rivendell while his father Arassuil dwelt in the wild.

Arathorn became the Chieftain of the Dúnedain in 2784 at the death of his father.

His rule was largely an uneventful one, as the Dúnedain and other peoples of Eriador were recovering from the Long Winter.

At the southern fringes of his realm great events occurred: the Dwarven King Thrór was slain by Azog, an Orc of Moria. From 2793–2799 the War of the Dwarves and Orcs was fought all through the Misty Mountains, as Dwarves from all Seven Houses gathered to avenge Thrór, the Heir of Durin.

After the War ended in 2799 at the Battle of Nanduhirion Eriador was safe from Orcs for many years, which meant that the Rangers of the North had a relatively safe period, asides from fighting the occasional Orc refugee. Most Orcs fled south to Rohan, trying to settle the White Mountains.

Arathorn died in 2848 when he was attacked by wolves, and he was followed by his son Argonui.