The Irish singer and activist said he could not share the honor with Suu Kyi, who has come under heavy criticism for not speaking out against alleged ethnic cleansing of Myanmar's Rohingya population.
"Aung San Suu Kyi was extravagantly welcomed to this city, and I was a participant to that ... and it turned out that she's a killer, and I don't want to be on the same list as what the UN described as a genocide," Geldof said in Dublin before returning the award Monday.
"I don't want to give this up, I'm really proud of it, you know, and you know I get handed things by states and cities around the world, but I'm a Dub [Dubliner] and this meant very much to me ... I don't want to do it, but it's the most I can do and the least," added the former Boomtown Rats singer and founder of 1985 charity concert Live Aid.
Suu Kyi faces international criticism
Suu Kyi has repeatedly denied accusations of ethnic cleansing and human rights abuses against the Rohingya.
Awards called in question
Suu Kyi was awarded the "Freedom of the City of Dublin" in 1999 while under house arrest, later receiving it in person in 2012.
Geldof, whose star-studded 1985 Live Aid concert raised funds for Ethiopian famine relief, received the award in 2006.