"To Pat," he wrote, "together we will win."
The $500,000 commitment, made by Manning and his wife, Ashley, is the largest gift so far to the Pat Summitt Foundation, which formed soon after the Lady Vols coach emeritus announced her Alzheimer's disease diagnosis in 2011.
The four-time NFL MVP and current Denver Broncos quarterback was joined at the presentation by Tyler Summitt, who co-founded the organization with his mother, and foundation board chair Jim Haslam.
Pat Summitt spoke briefly, expressing her gratitude.
"This is a special, special day and it belongs to all of you as well as us," she said. "And obviously, Peyton, you're the best."