Which Roman emperor was nicknamed “the Philosopher” because of his Stoic writings?

The Roman emperor nicknamed “the Philosopher” for his Stoic writings was Marcus Aurelius.

Marcus Aurelius ruled from 161 to 180 CE, first jointly with Lucius Verus and later alone. He is especially associated with Meditations, a collection of personal reflections written mainly in Greek while he was on military campaigns and at the frontier.

The work was not designed as a public biography or a formal philosophical treatise. It records reminders about self-discipline, duty, impermanence, and living according to reason. Marcus Aurelius was also an active ruler who faced wars against Parthia and Germanic peoples, as well as the Antonine Plague. His philosophical reputation should not obscure his role as an imperial administrator and commander.