As of 0017 GMT, the Nikkei was down 1.5% at 37,222.21.
The index was pulled down by a majority of stocks going ex-dividend, resulting in a loss of around 300 points, strategists said.
The broader Topix slipped 1.63% to 2,769.7.
Automakers Toyota Motor slumped 4.47%, making it the worst performer in the Nikkei, while Honda Motor dropped 4.26%. Both followed the overnight declines of General Motors and Ford.
Earlier this week, Trump declared his plan to implement a 25% tariff on imported cars and light trucks, effective from April 3, while the duty on auto parts begins on May 3. Investors also braced for a wave of reciprocal tariffs that Trump plans to unveil on Wednesday, although he has hinted there may be room for flexibility.