Keenan's manager, David Graham, tells TMZ ... Keenan had open heart surgery on Dec. 15 but there were complications from the procedure and he was put on life support. We're told Keenan died Thursday afternoon in a Chicago hospital after being taken off life support.

Keenan had a huge following on social media ... we're told he was one of the first YouTube influencers to rack up over 500 million views on his channel.

50 Cent actually helped take Keenan's career to the next level ... they did a surprise collab on the rapper's "Down On Me" song, opening the door for Keenan to work with other celebs.

Keenan's Rolodex was pretty impressive ... he also worked with Justin Bieber, Ariana Grande, Katy Perry, David Guetta, Britney Spears, Jason Derulo, LMFAO and Tyra Banks, just to name a few.

Keenan was diagnosed with a rare genetic disorder when he was a baby, Maroteaux–Lamy syndrome, a progressive condition that causes organs and tissues to enlarge, become scarred or inflamed. Some people who have Maroteaux-Lamy syndrome can also have heart issues, which appears to be the case here.

He was 27.