Frates -- who served as team captain in 2007 -- was diagnosed with Lou Gehrig's disease in 2012 ... and 2 years later, became the face of the worldwide campaign that raised more than $220 million for ALS research.

Frates didn't create the Challenge but was one of the first to do it -- and he's credited with being one of the most important factors in the Challenge going mainstream.

Huge names from Bill Gates to Lady Gaga to Alex Rodriguez participated in the fundraiser, which required people to dump freezing-cold buckets of water on their heads.

"Pete was an inspiration to so many people around the world who drew strength from his courage and resiliency," his family said in a statement.

"A natural born leader and the ultimate teammate, Pete was a role model for all, especially young athletes, who looked up to him for his bravery and unwavering positive spirit in the face of adversity."

"He was a noble fighter who inspired us all to use our talents and strengths in the service of others."

Frates had a successful career as an outfielder for the Eagles -- he had 11 home runs and set a modern BC record with 8 RBI in one game.

He went on to become the team's director of baseball operations in 2012.

The family is accepting donations to the Peter Frates Family Foundation in his honor.

"The Frates family wishes to express its sincere gratitude for the abundant love, kindness, and support we have been the recipients of during the past eight years."