CNN President Jeff Zucker resigned from the TV news organization, citing his failure to disclose a consensual relationship with a close colleague, and cutting short a nine-year tenure during which he helped transform the network’s role in the cable-news landscape.

Mr. Zucker’s abrupt departure comes as CNN is grappling with declining ratings as it also prepares to jump into the streaming wars with a new subscription service, CNN+, under future owner Warner Bros. Discovery.