The Irish screen veteran, 72, has been undergoing chemotherapy for Hodgkin's disease at University College Hospital, London, after falling ill two months ago.
He is expected to be discharged in time to work on the third Potter film, The Prisoner of Azkaban.
Harris, already a renowned actor, became known to a new generation of film fans through his role as Professor Albus Dumbledore in Harry Potter and the Philosopher's Stone - the first movie based on JK Rowling's tales of the boy wizard.
He had just finished work on the second movie, Chamber of Secrets, when he became ill this summer.
Hodgkin's disease - a form of cancer that affects the lymph glands - was diagnosed when Harris went into hospital with a severe chest infection in August.
His agent, Sharon Thomas, told BBC News Online he was "responding well to treatment".
"He will be out of hospital very soon," she said.
Seven-film contract
Harris built his reputation on his commanding performances in films such as A Man Called Horse and This Sporting Life.
But he was also renowned for his hellraiser lifestyle and heavy drinking.
He admitted his wayward nature almost lost him the role of Harry Potter's wise old wizard headmaster.
He and the rest of the cast were asked to sign up for a seven-movie deal - the number of books JK Rowling has always said she intends to write.
"The thought of doing seven films was intimidating," he said at the time.
"I'm rebellious by nature and felt it could be rather difficult to handle."
The star did sign up, however, and is due to start filming the third movie early next year.
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