"In the face of unexpected violence, most of us freeze. We crumble. We pray. We cry. But a select few stand up, join up and take the fight to the enemy, in the name of justice," he wrote, along with a minute-long video.
"This September 11th I am doing something I invite all of you to do as well. I'm posting a video to #ThankAVeteran," he continued.
The "Jurassic World" star is honoring Mike Day, a former Navy Seal that Pratt got to know recently.
But Day is moving forward with his life and will compete in the Kona Ironman World Championships later this year.
Pratt is donating $10,000 to his cause, Carrick Brain Center in Dallas, Texas.
Pratt would like tohers to honor a veteran they know, using the hashtag: #ThankAVeteran.
"I invite you to shoot your own short video thanking someone you know who's worn the uniform and defended our country," he added.