The 66-year-old Canadian expat made the announcement to staff at ABC News today.
An excerpt from an AP story on CTV.ca:
Jennings, 66, has been feeling ill for the past several months and was replaced Saturday on coverage of the pope's death by anchor Charles Woodruff. He last anchored "World News Tonight" on Friday.
Jennings informed ABC News staff of the diagnosis Tuesday morning and said he will anchor the broadcast when he feels up to it over the next few months as he begins chemotherapy.
"There will be good days and bad, which means some days I may be cranky and some days really cranky," he told ABC News employees in an e-mail.