Joao Carvalho was taken to hospital after being beaten in a fight at the National Stadium in Dublin by Charlie Ward, a team-mate of UFC champion Conor McGregor.
The 28-year-old underwent emergency brain surgery and was in a critical condition for 48 hours.
However, a statement from his Vitor Nobrega team confirmed he had died.
Irishman McGregor, who was at ringside for the fight, said Carvalho "was a hell of a fighter and will be sorely missed by all".
"The fight was so back and forth that I just can't understand it," McGregor added.
"Nobody involved in combat sports of any kind wants to see this.
"It is such a rare occurrence that I don't know how to take this.
"To see a young man doing what he loves, competing for a chance at a better life, and then to have it taken away is truly heartbreaking."
Total Extreme Fighting, which organised the event, said it was "deeply saddened" by Carvalho's death.
"We extend our most sincere condolences to the family of Joao Carvalho and his team-mates in Team Nobrega," said Cesar Silva, the chief executive of Total Extreme Fighting.
"We'd also like to thank all sports fans for their concern and support.
"We will give whatever support we can to Joao's family."
John Kavanagh, who coaches McGregor, was in Ward's corner during the fight and asked people to respect the privacy of the Portuguese fighter's family.
Kavanagh also works with the Irish Amateur Pankration Association (IAPA), which represents MMA in the country, and said the group will be working with Total Extreme Fighting to establish "all relevant facts and event processes" in the incident.
A Portuguese mixed martial arts (MMA) fighter has died after an event in the Republic of Ireland on Saturday.