Sarkic, a Montenegro international, joined the Championship club from Wolves in August 2023 and made 33 appearances last season.
He last played for his country in a 2-0 friendly defeat by Belgium on 5 June.
The Montenegro Football Association (FSCG) said Sarkic died suddenly on Saturday morning, with local media reporting he collapsed at his apartment in Budva.
The FSCG described Sarkic as a “beloved member” of the national squad and an “eternally smiling guy”.
“His untimely death has caused great pain to the Sarkic family,” it added.
Millwall said they were “completely devastated”.
“Everyone at the club sends their love and condolences to Matija’s family and friends at this immensely sad time,” the Lions said.
Sarkic, who was born in Grimsby, played youth-team football for Belgian side Anderlecht before joining Aston Villa.
He had loan spells with Wigan, Stratford Town, Havant & Waterlooville and Livingston before a permanent move to Wolves in 2020.
Loans at Shrewsbury Town, Birmingham City – where he made 23 appearances on loan during the 2021-22 season – and Stoke City followed before his move to Millwall.
Villa said on X: “Everyone at Aston Villa is deeply saddened.
“Matija joined our academy in 2015 and spent five years with the club, during which time he made his full international debut for Montenegro, before departing in the summer of 2020.
“The thoughts and condolences of all at the club are with his family and friends at this incredibly sad time.”
The Football Association, the Professional Footballers’ Association and the English Football League also offered their condolences to Sarkic’s family and friends.
BBC
