The 28-year-old was one of the key players in the Jambos' stirring title challenge last term and, after running down his contract, he was made an offer to stay that would have seen him become the highest-paid player at the club.
But Hearts issued a statement on Friday announcing that Devlin, who was part of Australia's World Cup squad, had not given them a definitive answer ahead of their stated deadline.
The Australian, who has been linked with Premiership rivals Rangers and Major League Soccer clubs in America, departs Tynecastle as Hearts' second-highest overseas appearance holder behind former captain Marius Zaliukas.
"Saying goodbye is never easy, especially to a club that has meant so much to me," Devlin said in an Instagram post on Friday evening. "After five unforgettable years, the time has come for me to say goodbye to Heart of Midlothian Football Club.
"This club has been so much more than just the place where I played football. It became my home, and I'm grateful for every moment.
"A huge thank you to every staff member, coach and teammate I've had the privilege of working with. I've made friends for life, and I'm grateful for the memories we shared.
"Playing for Hearts has been everything I dreamed of and more. European nights, derby wins, and the incredible season we've just had are memories I'll cherish forever.
"Hearts also gave me the opportunity to live out my dream of playing overseas and representing my country at two World Cups, something I'll always be thankful for.
"This wasn't an easy decision, and it certainly wasn't one I took lightly. I hope you can understand. Thank you to everyone at the club for believing in me and allowing me to be part of something so special.
"Hearts will always hold a special place in my heart. I wish everyone connected with the club nothing but success in the future."
Devlin joined Hearts in August 2021 and made 176 appearances for them, scoring eight goals.
