The Reservoir Dogs and Boardwalk Empire star, 63, had worked as a firefighter many years before the attacks in 2001.
After the Twin Towers came down he headed to Ground Zero to help with the search and recovery effort.
Speaking on the WTF podcast, he said he later found he "was depressed" and "anxious".
Buscemi explained that the day after the attacks he called his old New York fire house several times but could not get hold of anyone, so he went down to the site and found his former company, Engine 55, working there.
He said: "I asked if I could join them. I could tell they were a little suspicious at first, you know, 'What are you doing here?' But I worked with them that day."
Read more: The amazing story of Steve Buscemi and 9/11
Buscemi helped with the effort for a few more days and revealed that it had a big impact.
"I haven’t experienced any health issues, and I get myself checked out, but definitely, yeah, post-traumatic stress? Absolutely," he said.
The Fargo star went on: "I was only there for like five days, but when I stopped going and tried to just live my life again, it was really, really hard.
"I was depressed, I was anxious, I couldn’t make a simple decision. All those things.
"It’s still with me.
"There are times when I talk about 9/11 and I’m just right back there. I start to get choked up and I realise, ‘Ah, this is still a big part of me’."
The star said he was "so grateful" that he had access to the site and the opportunity to help.
Buscemi is an executive producer of Dust: The Lingering Legacy Of 9/11, a documentary about health issues among those who were at Ground Zero.
Watch: Steve Buscemi pens emotional account of experience on 9/11