A majority of senators - 57 to 43, including seven Republicans - voted to convict Mr Trump, 10 votes short of the 67 required for conviction.
After his acquittal, Mr Trump released a statement denouncing the trial as "the greatest witch hunt in history".
This was Mr Trump's second impeachment.
If he had been convicted, the Senate could have voted to bar him from running for office ever again.
After the vote, the senior Republican in Congress, Senator Mitch McConnell said Mr Trump had been "responsible" for the assault on the Capitol and called it a "disgraceful, disgraceful dereliction of duty".
Earlier, he voted against conviction, saying it was unconstitutional now that Mr Trump was no longer president. Mr McConnell was instrumental in delaying Mr Trump's trial until after he left office, on 20 January.
However, Mr McConnell warned Mr Trump could still be held liable in court.
"He didn't get away with anything yet. Yet. We have a criminal justice system in this country, we have civil litigation and former presidents are not immune from being [held] accountable by either one," he said.