Garcia clinched his world title in 2000 and triumphed in the end-of-year Triple Crown on six occasions.
Currently being treated in hospital in Oregon, the WSL paid tribute to the walk of fame surfer.
“With heavy hearts we confirm that Sunny Garcia is in the ICU in the hospital,” the WSL posted in a statement on Twitter.
“Sunny has always been a great champion of surfing, both in and out of the water. Our prayers are with him and his loved ones at this deeply challenging time.”
10 biggest sports contracts on the planet
Show all 11
Garcia was the second ever surfer to surpass the $1 million mark in career prize money.
He is also known for his involvement with charities for autism and cystic fibrosis.
Fans and fellow professional surfers have come out in support of Garcia, with former world champion Derek Ho pleading with him to "hang in there" in a since deleted post.
The 1993 world champion wrote: “Hang in there … [it’s] not your time! Only God knows! … our prayers are with you!!!”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies