Yonhap news agency said the Korea Institute for Defense Analyses (KIDA) had revised its figures after seeing a related Pentagon document.
It now estimated that Pyongyang had up to 50 medium-range missile launchers and 150 for short-range projectiles.
Seoul had previously estimated the North had up to 94 mobile launchers, Yonhap said, quoting a KIDA researcher as saying the US document showed North Korea had been expanding its missile programme despite its economic difficulties.
KIDA officials were not available to comment on the report, which came after the North's apparent decision to cancel an expected medium-range missile launch.
A US defence official said last week that two North Korean missiles that had been primed for firing as tensions on the peninsula flared in the run-up to the North's national celebrations on April 15 had been moved from their launch site.
US and South Korean officials had been worried that any test of the Musudan missiles would trigger a fresh surge in tensions, which escalated after the North carried out a nuclear test in February.