Arsenal hailed Jose Antonio Reyes' "special place" in Gunners' history after the 35-year-old died in a car crash.
Reyes' old club Sevilla acclaimed the former Spain striker as "beloved" in confirming his death.
The livewire forward formed part of Arsenal's 'invicibles' that swept to the 2004 Premier League title without suffering defeat.
"Everyone at Arsenal is devastated by the shocking news that our former player Jose Antonio Reyes has died in a traffic collision in Spain," read Arsenal's statement.
"Our thoughts are with all his family, friends and team-mates.
"Jose, who was part of the Invincibles team, was a hugely popular figure at our club and will always hold a special place in our hearts.
"He represented the club on 110 occasions, scoring 23 times and made memorable contributions to our 49-game unbeaten run. Rest in peace, Jose."
Sevilla confirmed Reyes' death, tweeting: "We couldn't be confirming worse news.
"Beloved Sevilla star Jose Antonio Reyes has died in a traffic collision. Rest in peace."
Atletico Madrid, meanwhile, said in a tweet: "The Atletico de Madrid family is in mourning. Our former player Jose Antonio Reyes has passed away. You will forever be in our hearts. Rest in peace."
Reyes made his senior debut for Sevilla aged just 16, quickly making a name for himself with a dangerous combination of pace and finishing ability.
Arsenal signed Reyes for £10.5million in January 2004, with the then 20-year-old helping the Gunners complete their remarkable unbeaten league season under Arsene Wenger, who expressed his condolences.
"I am devastated to hear the terrible news about Jose," said Wenger via a tweet issued by Arsenal.
"To his family and friends, all support from everyone in the Arsenal family. He will remain forever in our hearts."
Former Arsenal team-mate Freddie Ljungberg was among the first to offer a tribute to Reyes, posting on Instagram: "Numbed by the news about my former team-mate, Jose Antonio Reyes. Gone far too soon, my thoughts are with his family and friends."
Reyes enjoyed a loan stint at Real Madrid in 2006 before joining city rivals Atletico a year later.
A return to home club Sevilla followed in 2012, after which he moved on to play for Espanyol, Cordoba, and then for a short spell in China before returning to Spain at the start of this year with Extremadura.
Reyes won 21 caps for Spain between 2003 and 2006, and scooped the UEFA Cup and Europa League a combined five times in a decorated club career.
Of all his exploits across Europe, lifelong Sevilla fan Reyes still rated claiming silverware with his boyhood club as the most special.
Talking ahead of the 2014 Europa League final, in which Sevilla beat Benfica 4-2 on penalties, Reyes told orgullodenervion.com: "When I was small, I dreamt of winning a title with Sevilla.
"Winning a title is the nicest thing there is. You feel something special, like you've done a really good job.
"It's also great to see the fans so happy. In all the teams I've been at I've done very well. But Sevilla is my home."
Former Arsenal and France striker Thierry Henry admitted his distress in a heartfelt Twitter post.
"I'm devastated to hear the sad news about Jose Antonio Reyes," read Henry's Tweet.
"Wonderful player, superb team-mate and exceptional human being. I wish his family and friends continued strength and courage to get through this difficult time."
Current Arsenal manager Unai Emery, who coached Reyes for three and a half seasons at Sevilla from 2013-16, also expressed his profound sadness.
Tweeting in Spanish, Emery's words translated as: "Deeply sad and speechless after learning of this tragic news. I'll never forget your magic and your smile Jose, never. It was an honour to be by your side...a strong hug and all the spirit of the world to family, friends and clubs he represented. RIP friend."
A moment's silence will be observed ahead of the Champions League final between Liverpool and Tottenham, with UEFA president Aleksander Ceferin saying: "He had a glittering career and won numerous honours wherever he played and I am shocked and saddened that his life has been so tragically cut short."
