Routh, 59, was convicted by a jury last September of five criminal counts including attempted assassination after serving as his own defence lawyer at trial.
US District Judge Aileen Cannon handed down the sentence in Fort Pierce, Florida.
Prosecutors had recommended a life sentence while Routh had asked the judge, a Trump appointee, to impose a 27-year term.
Prosecutors said in a court filing that Routh's crimes "undeniably warrant a life sentence" because he had plotted the assassination for months, was willing to kill anybody who got in the way and has expressed neither regret nor remorse.
In a court filing, Routh denied that he intended to kill Trump, and said he was willing to undergo psychological treatment for a personality disorder in prison.
Routh suggested that jurors were misled about the facts of the case by his inability to mount a proper legal defence at trial.
Routh, who at the time of his arrest had resided most recently in Hawaii after previously living in North Carolina, also was convicted of three illegal firearm possession charges and one count of impeding a federal officer during his arrest.
Secret Service agents spotted Routh hiding in bushes a few hundred metres from where Trump was golfing at Trump International Golf Club in West Palm Beach on September 15, 2024.
Routh fled the scene and left behind an assault-style rifle but was later arrested.
The incident occurred two months after a bullet fired by a gunman grazed Trump's ear at a campaign rally in Butler, Pennsylvania.
Both incidents came in the run-up to the November 2024 election in which Trump regained the presidency after having been defeated four years earlier by Democrat Joe Biden.
Trump, a Republican, turned the attempted assassinations into a campaign issue, saying the US Justice Department under Biden could not be trusted with investigations.
Prosecutors said Routh arrived in South Florida about a month before the incident, staying at a truck stop and tracking Trump's movements and schedule.
Routh carried six mobile phones and used fake names to conceal his identity, according to trial evidence, and prosecutors said he lay in wait in thick bushes for nearly 10 hours on the day of the incident.
Investigators on the scene found the assault-style rifle, two bags containing body armour-like metal plates and a video camera pointed at the golf course.
Routh pleaded not guilty in the case but fired his lawyers and opted to represent himself at trial despite lacking any formal legal training.
His meandering opening statement touched on topics including the origin of the human species and the settlement of the American West before he was cut off by Cannon, who warned him against making a mockery of the courtroom.
Routh's defence strategy focused on what he described as his nonviolent nature but he offered little pushback as a parade of law enforcement witnesses detailed the evidence in the case.
Prosecutor John Shipley told jurors that Routh's plot was "carefully crafted and deadly serious," adding that without the Secret Service's intervention "Donald Trump would not be alive".
After the jury read the verdict, Routh appeared to try to stab himself with a pen several times and had to be restrained by US marshals.
His daughter yelled in court that her father had not hurt anyone and that she would get him out of prison.
Trump lauded the verdict in a post on his Truth Social site, writing, "This was an evil man with an evil intention, and they caught him".
