An official medical bulletin described the 81-year-old head of Europe's longest-reigning royal family as "stable", as he was put on a respirator at the Riviera principality's Cardio-Thoracic Centre.
Rainier was transferred to intensive care on Tuesday after developing a sudden respiratory infection and "cardiac and kidney insufficiency", a palace statement said.
The prince had been in the hospital for more than two weeks to treat an infection of the bronchial tubes and lungs. Heart problems complicated the initial infection, requiring "important treatment and a rigorous medical surveillance", the palace said.
The brief medical bulletin gave no prognosis for the prince, who has been in failing health for years and in and out of the hospital.
Rainier's film-star wife, Grace Kelly, died in a car crash in 1982.
Rainier's bachelor son and heir, Crown Prince Albert, returned from abroad to be at his father's bedside, as did his elder daughter Princess Caroline. Princess Stephanie is also at the hospital.
Father Philippe Blanc, the local curate, visited the prince today and Archbishop Bernard Barsi of Monaco also visited the hospital.
Rainier has ruled tiny Monaco nestled on the Mediterranean coast between Italy and the French Rivieras, and famous for its casinos, Formula One Grand Prix and tax breaks that attract the rich and famous since 1949.
Albert, Rainier's only son, is in line to succeed his father, due to a constitutional amendment in 2002 allowing him to reign despite his lack of heirs.