The two-time Oscar winner was the main speaker at the prestigious institution's commencement ceremony for students completing degrees.
He received an honorary arts degree himself.
"The truth to some is no longer empirical," he told an audience of more than 9,000 people.
"It's no longer based on data nor common sense, nor even common decency."
Telling the truth is no longer the benchmark for public service, he said.
"Truth is now considered malleable, by opinion, by zero sum end games."
Hanks also told the graduates they had a choice to be one of "one of three types of Americans, those who embrace liberty and freedom for all, those who won't, or those who are indifferent".
But he said that the responsibility to uphold the truth belonged to everyone.
"The effort is optional. But the truth, the truth is sacred. Unalterable. Chiselled into the stone and the foundation of our republic."
The 66-year-old's speech was not without its lighter moments as he joked about his lack of academic credentials.
"I don't know much about Latin, I have no real passion for enzymes and public global policy is something I scan in the newspaper just before I do the Wordle," he said, referring to the popular web-based game.
Hanks asked graduates not to be "embittered" by the fact he was receiving a degree "without having done a lick of work, without having spent any time in class, without once walking into that library".
But he made a "damn good living playing someone who did", he added, referring to the fictional Harvard professor Robert Langdon, whom he played in the films The Da Vinci Code, Angels & Demons and Inferno.
"It's the way of the world, kids," he said as his audience laughed.
Hanks, who was one of six people to receive honorary degrees on Thursday, was also given a Harvard volleyball in reference to one of his best-known roles in the film Cast Away.
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