Aaron Rajman was fatally shot during Monday night's apparent home invasion in the coastal city of Boca Raton.
Palm Beach County sheriff's office says a fight broke out after a group of suspects entered the 25-year-old featherweight's home.
Mr Rajman died from gunshot wounds. The assailants, whose motive is currently unknown, fled in a vehicle.
Dave Zalewski, a Miami fight promoter, told the Sun-Sentinel newspaper: "I'm just literally at a loss for words. Everyone I talked to is sickened.
"He was the most humble guy around, he never talked bad about people."
Neighbour Luciana Silva told local news station WPEC-TV that shortly before his death the victim spoke to her children to wish them a happy Independence Day.
Mr Rajman, an Orthodox Jew, was known for entering the arena wearing tzitzit and a yarmulke.
Ranked 26th in his class in Florida, he made his professional MMA debut three years ago and had a 2-2 record, according to MixedMartialArts.com.
As an amateur, Mr Rajman won eight fights and lost one.
"He was very strong in his faith, and he was a very good person," Rabbi Zalman Bukiet told local news station, WPTV.
"This was a special young man who had a heart of gold.
"It's a tragedy and extremely painful for them and painful for everybody that knew him."
A post on the MMA South Florida Facebook page read: "The world lost another warrior today. RIP Aaron Rajman. You will be missed."
A mixed martial arts fighter has been killed by intruders in the US state of Florida, police say.