Chris Wallace, the unflappable veteran Fox News anchor who has held US presidents' feet to the fire and moderated combative political debates, announced Sunday he is leaving the network to "try something new".

"After 18 years I have decided to leave Fox," the 74-year-old reporter, considered one of the most trusted television news anchors in America according to polling, said on his final hosting of Fox News Sunday.

"I want to try something new, to go beyond politics to all the things I'm interested in. I'm ready for a new adventure," he said, without elaborating.

Wallace, the son of famed "60 Minutes" journalist Mike Wallace, has interviewed every sitting US president since George H.W. Bush, and grilled foreign leaders from Russia's Vladimir Putin to Emmanuel Macron of France.

He has been seen as a steady, balanced voice amid the turbulence at Fox, where conservative hosts have gained prominence and earned criticism for their allegiance to Donald Trump and opposition to Democrats.

Trump, as president from 2017 to early 2021, routinely sought loyalty from Fox personalities but Wallace refused to buy in.

He said that from the day he began at Fox, he has been free to report on stories he sought to cover, and "to hold our country's leaders to account".

"The bosses here at Fox promised me they would never interfere with a guest I booked or a question I asked, and they kept that promise," he said on his final show.

Wallace in 2016 became the first Fox News reporter to moderate a presidential debate, when he fired tough questions at Trump and his rival Hillary Clinton in the final faceoff of their combative campaign.

US lawmakers, many of whom have been interviewed multiple times by Wallace, expressed shock and dismay at his sudden exit.

"Chris Wallace's departure from Fox News is another blow to a country that can ill-afford to lose principled journalists to opinionists and angertainers," Democratic congressman Dean Phillips said on Twitter.