"You want me to be afraid," he wrote in the letter. "You lost."
Leiris had been in love with his wife, 35-year-old Helene Muyal-Leiris, for 12 years. She was among the 89 people killed by terrorists in the Bataclan theater Friday. Now a widowed father, Leiris expressed his grief in a powerful letter he posted to Facebook in French, which has been shared more than 200,000 times since Monday.
He explained that he will not hate the terrorists because that is what they want.
"Therefore I will not give you the gift of hating you," he wrote. "You have obviously sought it, but responding to it with anger would be to give in to the same ignorance that has made you what you are."
Muyal-Leiris also leaves behind their young son, 17-month-old Melvil. Her husband wrote that he will raise Melvil to feel the same way he does: unafraid and free of hate.
"Every day of his life, this little boy will insult you with his happiness and freedom because you don't have his hatred, either."
The letter was translated into several languages, including English, which Leiris read aloud for BBC News in a powerful video.
The Associated Press contributed to this report.