Kenneth Iwamasa, 61, repeatedly administered the drug to the actor in the weeks leading up to his death, prosecutors said in legal documents, without medical training or safe equipment.
During this period in October 2023, he found Perry unconscious at least twice - and on one occasion, saw him "freeze up" and lose the ability to speak. Despite these "clear warnings", they said, he continued.
Judge Sherilyn Peace Garnett, sitting at a federal court in Los Angeles, also sentenced Iwamasa to two years of probation and a fine of $10,000 (about £7,400).
"You were privy to his struggle with addiction," Judge Garnett said. "Your conduct was reckless, not just on the day of his death but in the days leading up to his death."
However, she said there was "no hard evidence" he acted "with malicious intent, though some would disagree".
"I am so sorry to all of you," Iwamasa said in court, turning to face the Perry family. "I'm just so sorry to have done illegal acts I will forever regret. I will take that to my grave."
His lawyer, Alan Eisner, had argued for a six-month prison term with six months of home confinement, emphasising he was acting at the direction of a famous boss who had much more power.
"His loyalty to Mr Perry was paramount," Mr Eisner said in court. "He worshipped Mr Perry, he looked up to Mr Perry. All he did was please and accommodate Mr Perry."
In a filing before the hearing, defence lawyers said Iwamasa could "not simply say no" to his boss - and "that inability had tragic consequences".
But in their own letters submitted to the court, Perry's family members made it clear they blame Iwamasa for the star's death. He was a friend for 25 years, they said, a man they thought would help the actor maintain sobriety.
'My son paid the price'
"Matthew trusted Kenny. We trusted Kenny. Kenny's most important job - by far - was to be my son's companion and guardian in his fight against addiction," wrote Perry's mother, Suzanne Morrison. "We trusted a man without a conscience, and my son paid the price."
The actor's sister Madeline Morrison said Iwamasa had spoken at his funeral. "The person responsible for my brother's death stood up and addressed the people who loved him most," she wrote. "He tainted our final memories of saying goodbye."
In court, Lisa Ferguson, Perry's business manager and estate executor, described the assistant as "the monster who killed" the star.
After Perry's death, Iwamasa "continued his wrongdoing by destroying evidence and taking other steps to cover his tracks and obstruct justice", prosecutors said.
However, after investigators executed a search warrant at his home in January 2024, they said Iwamasa then accepted responsibility and agreed to co-operate in the government's investigation.
He was the first of five defendants to reach a deal with prosecutors, pleading guilty in August 2024 to one count of conspiracy to distribute ketamine, resulting in death.
Iwamasa began working for Perry in 2022. The star was paying him $150,000 (about £112,000) a year to live at his home in Los Angeles and act as his assistant.
The Friends actor had been taking the surgical anaesthetic ketamine legally for depression, an increasingly common off-label use in the US, after years of struggling with addiction.
But he started to want more than his doctor would give him legally.
In the final days of Perry's life, Iwamasa was injecting him six to eight times per day, legal filings said. On 28 October 2023, he injected the star with a shot of ketamine at about 8.30am and then again at 12.45pm.
Forty minutes later, Perry asked Iwamasa to prepare the jacuzzi and told him to "shoot me up with a big one", according to court documents. The assistant then left to run errands, only to find the star dead on his return.
The LA County Medical Examiner found ketamine was the primary cause of death, with drowning a secondary cause.
Iwamasa, the last of the five defendants to be sentenced, was described by prosecutors in court documents as an "enabler" and "drug supplier" to the star.
According to his plea agreement, he bought ketamine from another doctor, Salvador Plasencia, who taught him how to inject it.
Plasencia was jailed for two-and-a-half years at a hearing in December.
Another doctor, Mark Chavez, was sentenced later that month to eight months' home confinement after admitting to selling ketamine to Plasencia.
Both have surrendered their medical licences, according to the US attorney's office for California's central district.
Iwamasa also began buying ketamine from Erik Fleming, a drugs counsellor and former addict himself, who was an acquaintance of Perry's. He was jailed for two years earlier this month.
Fleming was getting the ketamine from dealer Jasveen Sangha, known as "the Ketamine Queen" in Hollywood, according to prosecutors. She received the longest sentence of the five, 15 years, at a hearing in April.
