The letter to Assad sent Tuesday expressed "enthusiastic congratulations" to the Syrian dictator who is suspected of using chemical weapons in the Syrian Civil War.
"I am pleased under your righteous leadership you have achieved great results in the struggle to defend your country's sovereignty and security," Kim's letter read.
Kim's title in the letter read "state chairman," according to Yonhap.
The North Korean leader stated he expresses his deepest sympathy and solidarity to the cause of justice for the Syrian government and people.
"I sincerely wish well for your health and that you will achieve greater results in your affairs," Kim's letter read.
The message did not include condemnations of U.S. airstrikes that were mentioned in a letter the North Korean leader sent to Assad in 2017, on the 71st anniversary of Syrian independence.
The omission could be connected to North Korean plans for a summit with U.S. President Donald Trump, to be held in late May or early June, according to Yonhap.
Debate is growing in South Korea whether North Korea is sincere regarding its intentions to "denuclearize."
Thae Yong-ho, the high-profile defector who fled Pyongyang's embassy in London, told reporters Tuesday Kim is likely to make "talks drag" for about two to three years without giving up nuclear weapons, the Chosun Ilbo reported Tuesday.
North Korea will go through phases of negotiation, including a declaration of nuclear disarmament, a period of nuclear disabling, then denuclearization through stages.
At each stage, North Korea will earn security assurances from the United States and South Korea, but will eventually not dismantle its nuclear weapons, Thae said.
Kim is engaging with the outside world because of sanctions, he added.
South Korea's summit with the North on April 27 also will not cover human rights, according to the unification ministry, Al Jazeera reported last week.