Najib, 72, was convicted on Friday of four counts of abuse of power and 21 counts of money laundering. The charges relate to the misuse of nearly 2.3 billion Malaysian ringgit from the state-owned investment fund 1Malaysia Development Berhad.
The decision was announced by a judge in Putrajaya, Malaysia’s administrative capital, following a lengthy trial that lasted seven years and involved testimony from 76 witnesses.
This is Najib’s second conviction connected to the 1MDB case. He has already been serving a prison sentence since 2022 after being found guilty in an earlier trial related to the same scandal.
Under the latest ruling, Najib received four separate 15-year sentences for the abuse of power charges and an additional five years for each of the money laundering counts. In line with Malaysian law, all sentences will be served at the same time.
The verdict came days after a court rejected Najib’s request to complete the remainder of his sentence under house arrest.
Friday's verdict comes after seven years of legal proceedings, which saw 76 witnesses called to the stand.
The verdict, delivered in Malaysia's administrative capital Putrajaya, is the second blow in the same week to the embattled former leader, who has been imprisoned since 2022.
He was handed four 15-year sentences on abuse of power charges, as well as five years each on 21 money laundering charges. The jail terms run concurrently under Malaysian law.
On Monday, the court rejected his application to serve the remainder of his sentence under house arrest.
But the former prime minister retains a loyal base of supporters, who claim that he's a victim of unfair rulings and who have showed up at his trials calling for his release.
On Friday, dozens of people gathered outside the court in Putrajaya in support of Najib
The 1MDB scandal made headlines across the world when it came to light a decade ago, embroiling prominent figures from Malaysia to Goldman Sachs and Hollywood.
Investigators estimated that $4.5bn was siphoned from the state-owned wealth fund into private pockets, including Najib's.
Najib's lawyers claim that he had been misled by his advisers - in particular the financier Jho Low, who has maintained his innocence but remains at large.=
But the argument has not convinced Malaysia's courts, which previously found Najib guilty of embezzlement in 2020.
That year, Najib was convicted of abuse of power, money laundering and breach of trust over 42 million ringgit ($10m; £7.7m) transferred from SRC International - a former unit of 1MDB - into his private accounts.
He was sentenced to 12 years in prison, but saw his jail term halved last year.
The latest case concerns a larger sum of money, also tied to 1MDB, received by his personal bank account in 2013. Najib said he had believed the money was a donation from the late Saudi King Abdullah - a claim rejected by the judge on Friday.
Separately Najib's wife, Rosmah Mansor, was sentenced to ten years in jail in 2022 for bribery. She is free on bail pending an appeal against her conviction.
The scandal has had profound repercussions on Malaysian politics. In 2018 it led to a historic election loss for Najib's Barisan Nasional coalition, which had governed the country since its independence in 1957.
Now, the recent verdicts has highlighted fissures in Malaysia's ruling coalition, which includes Najib's party United Malays National Organisation (UMNO).
Najib's failed house arrest bid on Monday was met with disappointment from his allies but celebrated by his critics within the same coalition.
Malaysia's Prime Minister Anwar Ibrahim called for politicians on all sides to respect the court's decisions.
Former Malaysian lawmaker Tony Pua told the BBC's Newsday programme that the verdict would "send a message" to the country's leaders, that "you can get caught for corruption even if you're number one in the country like the prime minister".
But Cynthia Gabriel, founding director of Malaysia's Center to Combat Corruption and Cronyism, argued that the country has made little headway in anti-corruption efforts despite the years of reckoning after the 1MDB scandal.
Public institutions have not been strengthened enough to reassure Malaysians that "the politicians they put into power would actually serve their interests" instead of "their own pockets", she told Newsday.
"Grand corruption continues in different forms", she added. "We don't know at all if another 1MDB could occur, or may have already occurred."