"I just think at the moment it's right that I'm also focusing on the pressing domestic challenges we have with the economy, and I think that's what people watching would reasonably expect me to be doing as well," Sunak said.
At the same time, the new UK PM told Sky News he was "very personally committed" to environmental issues.
Sunak said the UK should be "really proud of how we're doing" as "one of the countries that has decarbonized the fastest."
The COP27 UN climate change conference takes place in Egypt on November 7-8, two weeks after Sunak became prime minister.
Opposition Labour party leader Keir Starmer attacked the Tory leader's decision to "shun" the event, while Green Party leader Caroline Lucas said that Sunak's absence was a "shameful way to end the UK's COP Presidency".
The UK hosted the previous summit, COP26, in Glasgow in Scotland in 2021, with British politician Alok Sharma serving as president.