By 0022 GMT, the Nikkei fell 0.7%, trimming some of its more than 1% losses earlier in the session.
The broader Topix slipped 0.45% to 2,800.37.
Trump on Wednesday unveiled a 25% tariff on imported cars and light trucks starting next week, widening the global trade war he kicked off upon regaining the White House this year in a move auto industry experts expect will drive up prices and stymie production.
Toyota Motor lost 2.5% and Honda Motor was down 2.1%, while Nissan Motor slipped 3.6%.
Subaru lost 5.6% to become the worst performer on the Nikkei. The index for automakers lost 2.26%.