In a blog post on the band's website, the drummer thanked fans for their support and described his ten years as an "unbelievable journey", reported Digital Spy.
Smith wrote: "After ten years of being a part of this unbelievable journey, it saddens me to say that I will be leaving Panic. This was not an easy decision to come to, but after a lot of thinking it became clear that this is what's right for me and the band.
"I love this band with all my heart, and getting to see it grow from four kids in my parents' garage to what it is now has been incredible. I loved it all. But, at a certain point, I realised that I wasn't able to be there for the band the way I wanted to be, and more importantly, the way they needed me to be."
Smith wrote: "A few years ago, when my addiction grabbed hold and wouldn't let go, they could have easily kicked me out and never looked back, but they didn't. Instead, they encouraged me to seek treatment and gave me the time I needed to get my life back in order.
"They helped me through those rough times, and now, on the other side, with two years sobriety from prescription medication, and working on a year sobriety from alcohol, I am nothing but grateful for all they have done.