The measures, which include the closures of most workplaces and schools and are called a "circuit breaker" by authorities, were initially set to run until May 4.
But the city-state has seen a sharp jump in cases in recent weeks fuelled by infections in cramped migrant worker dormitories, many of which are under government-ordered quarantines.
The city-state reported 1,111 new coronavirus cases on Tuesday, bringing total infections to 9,125, after a record daily jump of 1,426 cases on Monday.
"We will therefore extend the circuit breaker for four more weeks," Lee said in a televised speech.
Lee said the extended lockdown period would help bring community cases down decisively and make sure infections in migrant dormitories do not spread into the wider community.
"Then, provided we have brought the community numbers down, we can make further adjustments and consider easing some measures," he said.
Singapore's finance minister said at a media conference that the government would extend economic support measures, including wage subsidies, to help businesses offset the impact of the longer lockdown period at a cost of S$3.8 billion.
The World Health Organization's regional chief said on Tuesday that Singapore – which has the highest number of reported cases in Southeast Asia – is facing "very difficult challenges" from a recent surge in infections but has the healthcare system and risk management capacity to handle it.