In an interview broadcast Friday on "CBS This Morning," Barr said there is a "very specific criteria" to satisfy the legal definition of treason and those conditions were not met.
Last week, Trump tweeted that the investigation into his campaign amounted to treason.
Treason only occurs when a U.S. citizen, or a non-citizen on U.S. territory, wages war against the country or provides material support to a declared enemy of the United States. Nothing of that sort has been alleged.
Though Barr didn't agree with Trump's assertion, he did fault what he said was biased decision-making by a "small group at the top" of the FBI.
Barr said unnamed senior FBI officials fell into the trap of thinking they're acting in the best interests of the country when in reality, "what they're doing is really antithetical to the democratic system."
In an interview broadcast Friday on "CBS This Morning," Barr said he was "just trying to state the bottom line."
Critics have accused Barr of spinning the report's findings to favor President Donald Trump. In a public statement earlier this week, Mueller pointedly denied that his investigation exonerated Trump of wrongdoing. Mueller said that if he "had had confidence that the president clearly did not commit a crime," the report would have said so.
Mueller said Justice Department policy prevented charging the president with a crime.
Barr, in the CBS interview, said he disagreed with Mueller's assessment that the evidence didn't exonerate Trump. He said that Mueller "could have reached a decision as to whether it was criminal activity."
President Donald Trump is expressing his displeasure with special counsel Robert Mueller's motives after Mueller blunt dismissal of Trump's repeated claims that the Russia investigation had cleared him of obstructing justice.
The president also offered mixed messages on Russia's efforts to help him defeat Democrat Hillary Clinton in the 2016 campaign. Early in the day, Trump tweeted he had "nothing to do with Russia helping me get elected."
That was the first time he seemed to acknowledge that Russia tried to help his campaign. Then on the White House South Lawn, Trump told reporters: "Russia did not help me get elected." He said he got himself elected.