The pact calls for sharing intelligence about North Korea and its nuclear and missile programmes among other topics, Yonhap news agency said, citing an unnamed foreign ministry source.
It said the North’s long-range missile launch in April and its other military threats had highlighted the need to swap information.
A Seoul foreign ministry spokesman told AFP the agreement covers the “protection of classified information” and would be signed as soon as the two countries complete domestic procedures.
“The two governments will officially sign the deal as early as this week, or sometime next week at the latest,” it quoted a government source as saying, adding that Seoul’s cabinet approved the move Tuesday.
A Japanese foreign ministry official in Tokyo said the intention was to sign it “at the earliest possible time”.
Many older Koreans have bitter memories of Japan’s 1910-45 colonial rule and historical disputes still mar the relationship. South Korea needs the pact “because we have to use Japan’s intelligence assets, including its spy satellites and high-end surveillance aircraft”, said the South Korean official quoted by Yonhap.
“It is an undeniable fact that the existence of Japan is important for our national security,” the official said, citing the presence of US forces in Japan as well as in South Korea.