Heng Swee Keat, 54, was rushed to hospital where doctors attended to him, Lee Hsien Loong said in a Facebook post.
"We were having our weekly cabinet meeting just now when Heng Swee Keat suddenly collapsed," Lee said.
"The doctors in Cabinet (we have three) attended to him immediately. We called an ambulance, which took him to TTSH," he added, referring to the government-run Tan Tock Seng Hospital.
"A CT scan showed that he had had a stroke. Hope Swee Keat will be alright - he is a valuable member of my team."
Heng, who formerly headed the education ministry and central bank, is among the younger cabinet ministers considered by analysts and the media as someone who can succeed Lee.
"He is one of Singapore's finest sons, and a leader with much promise," said Tharman Shanmugaratnam, one of the city-state's two deputy prime ministers, on Facebook.
Home affairs minister K. Shanmugam said on social media he noticed that his cabinet colleague seemed "very tired" and had told Heng that he was "overworking so much that it will affect his health".
