The 50-year-old royal was rushed to hospital six days ago after suffering from pneumonia. The Belgian palace confirmed the news on Tuesday.
Prince Laurent, the younger brother of King Philippe, is said to be in a stable condition and could be in the coma for up to two days.
"An artificial coma would avoid the patient exhausting themselves by fighting against the life-support system," reported Belgian television channel RTL.
"Prince Laurent was voluntarily placed in a coma to give the antibiotics time to act against his pneumonia. This could last up to 48 hours."
Father-of-three Laurent was taken to the Cliniques Universitaires Saint-Luc hospital in Brussels last week.
His wife princess Claire, who was born in Bath, England, has been at his side every day since his hospitalisation.
The couple married in April 2003 and are parents to Princess Louise, 10, and nine-year-old twin sons Prince Nicolas and Prince Aymeric.
Belgian King Philippe visited his younger brother on Monday, while Queen Mathilde went to see the royal early on Tuesday.
The reigning monarchs, who ascended to the throne last summer after Albert II abdicated, were scheduled to go on an official trip to Madrid.
The duo cancelled their royal engagement following the death of former Spanish prime minister Adolfo Suarez, giving them more time to spend with the ailing prince.