Thaw and his wife Sheila Hancock's lives have been touched by cancer before.
The 59-year-old, known to a generation as Inspector Morse, is suffering from cancer of the oesophagus.
In a statement Thaw said: "I am receiving treatment for cancer of the oesophagus.
"As soon as this has been completed, I intend to return to work."
Fellowship
The actor, one of the best-loved figures on British television, came to fame as detective Jack Regan in 1970s police drama The Sweeney, playing Inspector Morse for the first time in 1987. He went on to appear in 33 Morse stories.
Thaw received a Bafta fellowship in May, after previously winning two Bafta TV awards - both for his role as the Oxford-based detective, who was killed off last year in an episode watched by 13 million people.
In 1995 he began appearing as Kavanagh QC and recently recorded a radio play as Captain Hook in Peter Pan.
The hard-working actor recently spoke recently of his desire to cut down his workload: "In the last few years I have made a conscious decision to slow down and I've not worked for a couple of months, quite deliberately."
Thaw and his wife Sheila Hancock's lives have been touched by cancer before.
Hancock's first husband, Alec Ross, died of the disease in 1971.
And their grandson Jack, six, suffered from a brain tumour last year although he is now said to be "out of the woods".
Ten years ago Sheila Hancock herself was diagnosed with breast cancer, but recovered.
"Sheila is very blase about it," Thaw has said.
"She has to have a check-up every year, but she doesn't make a big thing about it.
"They do say that after about seven or eight years it is unlikely the cancer will come back - but the thought is always with you."
Dennis Waterman, who co-starred in The Sweeney with Thaw, said he was confident he would beat the disease.
He said: "From day one he has been very positive. If there is anyone who is going to beat it then John will."