Sullenberger, 75, disclosed the diagnosis in a statement on his website and said he was getting treatment.
"For now, this means a name may not come easily to me, I forget a story I have recently told, or I don't sleep as well, but I am in the beginning of this long journey," he said on Tuesday.
The veteran pilot has been a vocal advocate for aviation safety since he landed a distressed US Airways jetliner on the Hudson River, saving all 155 on board in what experts called a masterful job under life-or-death pressure.
He became an international celebrity played by actor Tom Hanks in the 2016 film Sully, directed by Clint Eastwood.
In 2022, Sullenberger briefly served as the US ambassador to the International Civil Aviation Organization based in Montreal. Before he left, he raised concerns about attempts by regional airlines to reduce pilot requirements that he warned would weaken safety.
"I am deeply committed to aviation safety and security - to the safety and wellbeing of all who fly," he said at the time, adding that commitment predated the "Miracle on the Hudson" flight and would remain his focus as he returned to private life.
In 2019, Sullenberger testified before the US Congress in support of requiring pilots to get new simulator training before Boeing 737 MAX flights could resume following two fatal crashes.
"Our current system of aircraft design and certification has failed us," Sullenberger said.
Congress in 2020 approved sweeping legislation to reform how the Federal Aviation Administration certifies new planes.
The Centers for Disease Control and Prevention said in 2024 an estimated 6.9 million Americans age 65 and older had Alzheimer's and it was the sixth-leading cause of death among Americans age 65 and older.
