Aldo requested and was granted a release from his UFC contract, his coach and manager, Andre Pederneiras, told ESPN.
Aldo's retirement alone would not get him out of his UFC contract, which would be frozen in that instance.
The legendary fighter has been removed from the UFC's external official rankings, signifying he is no longer on the active roster.
Provided he was released from his contract, he would be free to compete for other promotions or in other combat sports like boxing.
Aldo (31-8) is arguably one of the top five fighters ever in mixed martial arts and easily within the top 10.
The Brazilian striker held the UFC/WEC featherweight title from November 2009 to December 2015, accumulating a division-record nine title defenses.
Aldo won 18 straight fights from 2006 to 2014, including victories over fellow luminaries like Urijah Faber and Frankie Edgar.
Aldo will cap his UFC career with a 21-7 record between that promotion and WEC, which was purchased and absorbed into the UFC in 2011.