Aoun's call on Wednesday came after he paid a visit to Geagea in his cell at the Defence Ministry.
"Keeping him in prison is an injustice ... I declare my solidarity with him until he is released," Aoun told reporters after the one-hour meeting.
"The visit lays the foundations for a new relationship within the democratic process we are witnessing today, and within the framework of letting go of the past which is now part of history," Aoun said at a news conference later on Wednesday.
He described the meeting with Geagea as "cheerful" and positive.
Aoun and Geagea, former leader of the now-defunct Lebanese Forces militia, fought a savage war for control of the Christian heartland in the final days of the 1975-90 civil war in which about 800 people were killed.
Aoun, who returned from 14 years of exile in Paris this month, said the meeting symbolised a new beginning.
The meeting, their first in more than 15 years, highlights the substantial changes in Lebanon's politics since the 14 February assassination of former prime minister Rafik al-Hariri and 20 others in a massive bombing in Beirut.