Kim made the remarks in a verbal message to Chinese President Xi Jinping, congratulating him on the successful completion of the Beijing Olympics, state news agency KCNA said in a summary.
North Korea and China are defending and advancing socialism, while "frustrating the undisguised hostile policy and military threat of the U.S. and its satellite forces" by strengthening strategic cooperation and unity, Kim said.
Kim praised the Games for making a mark on history, and said under Xi's leadership, China had persevered in the face of an "unprecedentedly severe health crisis and the hostile forces' maneuvers."
North Korea did not participate in the Games, which ended on Sunday.
In a previous letter from sports authorities in January, North Korea blamed "hostile forces" and Covid-19 risks for its not being able to attend.
North Korean athletes were not eligible to compete under their national flag after the country failed to send a team to the Tokyo Summer Olympics last year, citing Covid-19 concerns.
China has been North Korea's only major ally since the two signed a treaty in 1961.