The defence begins after Democrats wrapped up their presentation in the case for his removal on Friday.
Democratic congressmen who serve as prosecutors in the trial have laid out meticulous evidence over three days that they said proved Mr Trump had abused his power and obstructed Congress.
They alleged that he pressured Ukraine to dig up political dirt on Joe Biden, a domestic rival, and that he sought to hide the evidence from Congress, another impeachable offence.
President Trump and senior Republicans claim Mr Biden and his son Hunter were involved in a corrupt business scheme in Ukraine.
Here’s what happened this week, and what we can expect to see in the coming days, with the help of the BBC’s Anthony Zurcher.
How the prosecution case unfolded
Proceedings began on Tuesday with a tussle between Democrats and Republicans over the rules of the trial.
Republican Senate Majority Leader Mitch McConnell proposed a tight two-day limit for opening arguments by both sides, before extending it to three after protests from Democrats.
Mr McConnell delayed until next week debate over motions from Democrats to allow new witnesses to be called and fresh evidence submitted.
Democratic congressman Adam Schiff, the head of seven impeachment managers who serve as prosecutors, opened oral arguments to a packed Senate chamber on Wednesday.
Mr Schiff said the president’s actions were exactly what the founding fathers feared when they came up with impeachment – “a remedy as powerful as the evil it was meant to combat”, Mr Schiff said.
The impeachment managers walked the senators through testimony gathered during depositions and committee hearings last year, from government officials, that they say points to a scheme by Mr Trump and his advisers to lean on Ukraine to investigate the Bidens.
The managers interspersed their oral arguments with audio and video tape, using the president’s own words – including a now-infamous call with the president of Ukraine – in their effort to portray him as guilty.
They directly addressed the claims against the Bidens – a purposeful attempt to get on the front foot ahead of the president’s defence.
Representative Hakeem Jefferies, an impeachment manager, eschewed the founding fathers, choosing instead to quote the late Brooklyn rapper Notorious BIG.
“And if you don’t know, now you know,” he said – a famous line from the song. He stood next to a picture of the rapper mounted on an easel – a first for the Senate chamber.
On Friday, the managers tackled the obstruction of Congress charge.
The managers argued that Mr Trump’s refusal to allow certain members of his administration to answer questions from the House of Representatives was akin to hiding information from a grand jury investigation.
Anthony’s take on prosecution:
The Democrats were determined to squeeze every drop out of the opportunity they were afforded. Republican senators, who had to sit through the marathon sessions – Schiff, alone, would speak for hours at a time – criticised the presentation as repetitive, but repetitive was the point.
For three days the Democratic case against the president, broadcast during the daytime on US network television and gavel-to-gavel on cable news networks, was ubiquitous. It was unavoidable.
It may not be nearly enough to convince the requisite number of Republican senators, but the uninterrupted fusillade was as much geared toward public opinion as it was towards the people in the Capitol.
What will the defence say?
The president’s defence team is expected to focus on their accusations about the Bidens.
Responding to the impeachment managers’ case, Republican senators and members of the defence team said on Thursday that the Democrats had essentially invited them to do so.
“What I don’t understand is that, for the last five hours, it’s been a lot about Joe Biden,” said Jay Sekulow, the president’s personal lawyer. “They kind of opened the door for that response.”
“Today the House Democrats – perhaps unintentionally – they threw Joe Biden under the bus,” said Ted Cruz, Republican senator from Texas.
The Republicans will have the same amount of time to present their case as the Democrats did – 24 hours over three days. Then they will again tussle over whether to introduce new witnesses.
The defence submitted far less documentary evidence ahead of the trial – just seven pages of broad arguments, compared to the Democrats’ more than 100 pages of detailed evidence.
Anthony’s take on the defence:
While the Democratic presentation was an endurance test, the Trump team’s defence promises to be a much leaner affair. They benefit from starting with a commanding lead in the Senate vote-count and, in effect, only need to provide a modicum of cover for their party faithful.
Up until now, their strategy has been to offer a wide array of defences, allowing Republicans to pick and choose what to use. As with any trial, the burden is on the prosecution to construct a cohesive compelling narrative, while the other side need only poke holes and find weaknesses.
Trump’s lawyers promise to present a “robust” defence, which will probably include going on the attack – against Democrats and their motivations and against Joe Biden and his family. While they have the unfortunate luck of starting their case on a Saturday morning, by the beginning of next week they will have a much bigger audience and, at least in this round, the final word.