The package worth 2.2 trillion yen ($15.5 billion) will include help with corporate financing and easing of loan conditions at a government-backed lending institution, said Chief Cabinet Secretary Yoshimasa Hayashi.
"We will provide full support for small and medium-sized enterprises affected by the US tariffs," Hayashi told a press conference.
Japan, a key US ally and its biggest investor, is subject to the same 10 percent baseline tariffs imposed on most nations plus steeper levies on cars, steel and aluminium.
US President Donald Trump also announced 24 percent "reciprocal" tariffs on Japan in early April, but later paused them along with similar measures on other countries until early July, reports BSS.
Japan wants all levies on its imports announced by Trump lifted.