The 23-year-old collapsed in the ring on Saturday shortly after his WBC Latino Silver lightweight bout with Eduardo Javier Abreu in Argentina ended in a draw.
He was taken to hospital and underwent emergency surgery but died on Thursday.
“Rest in Peace, Hugo Santillan,” the World Boxing Council said in a tweet.
Santillan is the second boxer to die from injuries sustained in the ring this week after Dadashev’s death was confirmed on Tuesday.
The 28-year-old had been hospitalised with bleeding on the brain after his IBF light-welterweight fight against Subriel Matias was stopped at the end of the 11th round last Friday.
He underwent emergency surgery but failed to recover.