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.
In their closing statements, the Democratic House of Representatives lawmakers appointed to shepherd the process through the Senate warned that it would be dangerous to acquit Mr Trump.
“The stakes could not be higher because the cold, hard truth is that what happened on 6 January can happen again,” Representative Joe Neguse said.
“History has found us. I ask that you not look the other way,” Representative Madeleine Dean said.
However, Mr Trump’s lawyer, Michael van der Veen, called the proceedings a “show trial” and said the Democrats were “obsessed” with impeaching Mr Trump.
Senators initially voted for personal testimony, which would have delayed a possible verdict on Saturday. But after emergency consultations to avoid any hold-up, they changed their ruling to admit written statements only.
The flip-flop came after discussion of a phone call between the former US president and a top Republican official, Kevin McCarthy, while the riot was going on.
A Republican member of the House of Representatives, Jaime Herrera-Beutler, said Mr McCarthy had told her about the call on the day.
She said Mr McCarthy had implored Mr Trump to call off the rioters, but the president had wrongly blamed left-wing activists broadly known as “antifa”.
“McCarthy refuted that and told the president that these were Trump supporters,” Ms Herrera-Beutler said. “That’s when, according to McCarthy, the president said: ‘Well, Kevin, I guess these people are more upset about the election than you are.'”
Instead, the congresswoman’s statement was admitted in written evidence. (Courtesy BBC)