The 83-year-old, who remains the only coach to have participated in six World Cup tournaments with different countries, has been suffering since 2023 from Hodgkin's lymphoma, a cancer of the lymphatic system.
Parreira was "admitted to intensive care due to a lung infection," according to the Rio de Janeiro hospital where he was admitted Wednesday.
The facility said he is on a ventilator and that his condition is stable.
The Brazilian holds the record for the most World Cup appearance as head coach. In addition to his two stints leading the Selecao in 1994 and 2006, he coached Kuwait in 1982, the United Arab Emirates in 1990, Saudi Arabia in 1998 and South Africa in 2010.
Parreira's 1994 victory came in the United States, where the World Cup is presently being co-hosted along with Canada and Mexico.