Harris, who was standing between New York Sen. Chuck Schumer and President Joe Biden, reached over the president to shake hands with Trump, who was standing next to former New York City Mayor Michael Bloomberg, after Harris appeared to be prompted by Bloomberg.
The greeting came less than 12 hours after Harris and Trump met for the first time at a presidential debate in Philadelphia, hosted by ABC News.
Tuesday night's handshake between Harris and Trump ended an eight-year streak of no handshakes on the presidential debate stage. There are no rules or stipulations requiring a handshake before or after the debate.
Kamala Harris, Donald Trump shake hands at start of presidential debate
The president and vice president on Wednesday will be attending 9/11 anniversary ceremonies at Ground Zero; Shanksville, Pennsylvania; and the Pentagon. Trump will also visit Shanksville after the ceremony at Ground Zero.
"On this day 23 years ago, terrorists believed they could break our will and bring us to our knees. They were wrong. They will always be wrong," Biden said in a statement. "In the darkest of hours, we found light. And in the face of fear, we came together -- to defend our country, and to help one another."
"Today, our longest war is finally over. But our commitment to preventing another attack on our people never will be," he said.