He was only 20 years old.

Palfreyman -- who was in his 2nd year with the Ayr Centennials of the Greater Ontario Junior Hockey League -- collapsed while he and his team were in the locker room during the first intermission of their Ayr Farmers Mutual Global Invitational tournament game at the North Dumfries Community Complex.

Waterloo Regional Police tell us officers arrived at the arena after receiving a medical emergency call, adding that "members of the community assisted the individual until emergency services attended."

Palfreyman was transported to Cambridge Memorial Hospital but, tragically, he was pronounced dead. A cause of death has not yet been revealed.

Palfreyman's Ayr Centennials team is heartbroken over his passing ... saying in a statement, "Thank you all for the outpouring of love for our beloved captain and beautiful young man Eli."

"Words cannot describe how much it means to his family/Cens family. We are all broken but feel your arms around us ❤️ #RIP17 ❤️"

Palfreyman was just named team captain for this upcoming season ... and said last Friday he was honored to receive the title.

The Greater Ontario Junior Hockey League has postponed all games scheduled for this week "out of respect for Eli and his family."

North Dumfries Township Mayor Sue Foxton also ordered flags to be flown at half-mast as a tribute to the hockey player.