The Communist Party elected Lam general secretary at a morning session, the party said at a press conference, making him the most powerful figure in Vietnam's leadership structure.
Lam's ascension comes two weeks after the death of General Secretary Nguyen Phu Trong, Vietnam's most powerful leader in decades, who presided over the so-called blazing furnace crackdown on corruption.
In his first remarks as general secretary, Lam pledged to "speed up" the campaign that has seen numerous top officials and senior business leaders face charges.
"(I will) continue to speed up anti-corruption ... regardless of who the person is," Lam told the press conference.
"We have gained the trust of the people, and international friends, in our fight".
The party's general secretary is the most powerful figure in Vietnam's leadership structure, with the president occupying a largely ceremonial role that includes meeting foreign counterparts.
Lam became president in May after his predecessor resigned amid the vast anti-graft purge.
It was not immediately clear if Lam would simultaneously continue to serve as president.
Chinese President Xi Jinping sent a congratulatory message to To Lam, Chinese state broadcaster CCTV reported.
"To Lam has not been fearful of taking down some highly important people," Carl Thayer, emeritus professor at the University of New South Wales, told AFP.
"He is going to do it again."
- 'Blazing furnace' -
The new general secretary's predecessor Trong died two weeks ago in Hanoi "due to old age and serious illness", with Lam then informally handed the reins of power.
His formal move into the top job follows a long career with the secretive public security ministry, which deals with the monitoring of dissent and surveillance of activists in Vietnam, a one-party state.
Lam, 67, led the ministry from 2016 until he became president earlier this year.
While the "blazing furnace" campaign has proved popular with the public, analysts have warned the turmoil is threatening the country's reputation for stability.
The Communist Party has undergone a series of upheavals in recent months, with ministers, business leaders and two presidents all falling from grace as part of the vast campaign.
And with many fearful of being caught in its crosshairs, everyday transactions within business and government alike have slowed.
"Decision-making has slowed in Vietnam because people are afraid that they might cross the line and get them involved in accusations of corruption," said Thayer.
"To Lam has got to work that out."
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