The 45-year-old star was promoting his new movie Due Date in Hollywood when he was asked about Gibson's recent sacking from The Hangover sequel.
Downey Jr suffered with drug problems in the past and Gibson gave him a chance to get back into movies with the 2003 film Singing Detective.
Speaking about Gibson being fired, Downey Jr said to Hollyscoop: 'It doesn't mean anything.
'The great thing about Mel is that you know someone for 25 years, whenever someone asks you a question about him in the media it doesn't mean anything. I just talk to the guy and we have a friendship.'