Watt, who is the child of a firefighter, was born in Waukesha (Wis.) and played collegiately at Wisconsin.
The three-time NFL Defensive Player of the Year donated to a GoFundMe page created for Sun Prairie (Wis.) Fire Department Capt. Cory Barr, who was killed Tuesday evening.
A natural gas leak in Sun Prairie led to an explosion that injured 15 and killed Barr on Tuesday evening.
Barr leaves behind a wife, Abby, and 3-year-old twin daughters.
Being generous is nothing new for Watt, who was named the winner of the Walter Payton NFL Man of the Year Award. The award recognizes a player's on-field excellence and volunteer and charity work.
The 29-year-old also was named to TIME Magazine's annual list of the world's 100 most influential people in April.
Watt came to the aid of the city of Houston and parts of Southeast Texas in the wake of Hurricane Harvey by launching a campaign via social media that ultimately generated more than $37 million in disaster relief assistance.
He started the relief effort with $100,000 of his own money on Aug. 27 while Houston was being hit hard by the flooding. Watt had an initial goal of $200,000, meeting that target on YouCaring.com within two hours.
Watt also paid the funeral expenses for the 10 victims of the Santa Fe High School shooting in May.