The 3-time NFL Defensive Player of the Year made the surprising announcement on his social media page Tuesday morning ... saying he's grateful his two-month-old son, Koa, was able to watch his last-ever home game on Christmas.

"Koa's first-ever NFL game," Watt said in his announcement. "My last ever NFL home game."

"My heart is filled with nothing but love and gratitude. It’s been an absolute honor and a pleasure."

Stars from all over the sports world congratulated Watt on his legendary career, including Kevin Durant, who wrote, "True game changer and a legend. Much respect 99."

The former Wisconsin Badger was the 11th overall pick of the 2011 NFL Draft and played the majority of his 12-year career with the Houston Texans. Watt suited up for Arizona in his last two seasons.

Watt logged 111.5 sacks and 27 forced fumbles during his career ... and earned five All-Pro selections. He also won Walter Payton NFL Man of the Year in 2017.

The 33-year-old, who just had his first baby with Kealia Ohai Watt in October, may not be on the field after this season ... but his family will still be represented well in the NFL nonetheless -- as his brothers, Derek and T.J., currently play for the Pittsburgh Steelers.

J.J.'s final two NFL games will be against the Falcons this weekend ... and then the 49ers on Jan. 8.