In announcing his candidacy, Sestak, a Vietnam War veteran who also served in Iraq and Afghanistan, emphasized presidential accountability.
"What Americans want today is someone who is accountable and above self, above party, above any special interest … a president who has the depth of global experience to restore America's leadership in the world to protect our American Dream at home … and one who is trusted to restructure policies where too many see only the growth of inequity not of the economy," he said in a statement on his website.
In a video posted on the site, Sestak listed several priorities for his campaign.
"It now is truly one world, where destruction by climate change, contraction of our way of life by China and damage to our national security by corporations will happen no matter what we do, just by ourselves," he said.
“We must convene the world for two primary objectives: Putting a brake on climate change and putting an end to an illiberal world order’s injustices.”
Sestak served two terms in Pennsylvania’s 7th Congressional District and 31 years in the Navy. He was defeated in a 2010 U.S. Senate race by Republican Sen. Pat Toomey.
He said he joined the race late because his daughter had been receiving treatment for brain cancer.
“The decision to delay was so I would be there with Alex, our daughter, as the brain cancer she had courageously beaten at 4 years old returned this past year.” He said his daughter "has again overcome the single-digit odds” of beating the disease.
It’s unclear if Sestak will be added to the first Democratic primary debates on Wednesday and Thursday nights in Miami, where the candidates have already been split into two groups for the matchup hosted by NBC, MSNBC and Telemundo.