The patient, aged in his 40s, is in a critical condition in intensive care at the Royal Adelaide Hospital.
His condition declined while he was in hotel quarantine at the Tom’s Court medi-hotel.
The man is the first person to be admitted to ICU with coronavirus in the state since May 2020.
South Australia recorded one new case of COVID-19 today – a child who returned from overseas and has been in isolation since her arrival.
The state has vaccinated close to 30,000 people since the rollout began in February.