Franklin, 68, headlined the show from 1975-84.
"Actress Bonnie Franklin has been diagnosed with pancreatic cancer and is currently undergoing treatment," a family spokesman told "ET" in a statement Monday.
"She and her family remain extremely positive and thank everyone for their support and concern. Respect for her privacy is greatly appreciated."
Mackenzie Phillips, who played Franklin's daughter Julie on the show, took to Twitter Monday to offer her support.
"Sending out love and sweet prayers to my 'mom'. The incomparable Bonnie Franklin," Phillips tweeted.