"I formally and humbly request to be made a citizen of the Islamic State,” Hassan wrote in the letter to the self-proclaimed Caliph of the Islamic State, Abu Bakr al-Baghdadi.
Fox News obtained the letter and published its contents on Friday.
Hasan's lawyer, John Gallagin, said his client's request to join ISIS “underscores how much of his life, actions and mental thought process are driven by religious zeal."
Hasan, who killed 13 and injured 30 in his attack at Fort Hood in 2009, signed the letter with his name followed by SoA, short for Soldier of Allah.
"It would be an honor for any believer to be an obedient citizen soldier to a people and its leader who don't compromise the religion of All-Mighty Allah to get along with the disbelievers," Hasan wrote.