Soldiers try to enter the National Assembly building in Seoul on December 4 2024, after South Korea President Yoon Suk Yeol declared martial law.
Lee Sang-min said he was stepping down "in grave recognition of responsibility for failing to serve the public and the president well", according to the JoongAng Ilbo newspaper.
President Yoon Suk Yeol accepted his resignation, the paper said.
Lee and Yoon are among those being investigated for alleged insurrection following the declaration of martial law on Tuesday.
Yoon, who narrowly survived an impeachment motion in parliament on Saturday, was forced to rescind martial law after lawmakers voted down his decree.
Earlier on Sunday, Police arrested Kim Yong-hyun, who was defence minister at the time of the martial law operation.
Kim had already been handed a travel ban.