Tomo Milicevic has quit 30 Seconds To Mars, saying it's the "right thing" for him to do.
"Please believe me when I say that this is the best thing for me in my life and also for the band," he said in a statement on Twitter.
The guitarist hasn't yet given any other information on when or why he decided to leave the band.
He hasn't featured on the band's current tour since March, citing "personal matters".
The remaining members of 30 Seconds To Mars, Jared and Shannon Leto, are yet to comment.
"I don't really know how to explain in a note how I could have come to a decision like this," he said.
"Even though this is incredibly painful because of my attachment to, and love for everyone involved, I know that this pain will be fleeting for myself."
The 38-year-old thanked frontman Jared and drummer Shannon Leto "for allowing me the privilege for being a small part of their dream".
He added: "Even though I'm leaving the band, my spirit will never leave the family".
Tomo's departure leaves just the founding members in the band, brothers Shannon and Jared Leto.
They formed 30 Seconds To Mars in 1998, with Tomo joining in 2003.
In March, the band tweeted a statement saying Tomo was "taking a break from tour to deal with some personal matters."
We wanted to let you know that Tomo is taking a break from tour to deal with some personal matters. Thank you for respecting his privacy at this time. We will be continuing the Monolith Tour and are looking forward to seeing you all on the road very soon.— THIRTY SECONDS TO MARS (@30SECONDSTOMARS) March 16, 2018
Thirty Seconds To Mars shot to fame with their 2005 album A Beautiful Lie and more recently America, which includes songs Dangerous Night and Walk on Water.
Lead singer Jared Leto is also well known as an actor, having played The Joker in Suicide Squad and won an Oscar in 2014 for his role as a trans woman in Dallas Buyers Club.
He was joined on stage by Shawn Mendes at Radio 1's Biggest Weekend in Swansea in May, where they performed Rescue Me.
Newsbeat has contacted representatives for 30 Seconds To Mars for comment.