Tokyo's benchmark stock index rose above the 30,000-mark for the first time in around five months.
The Nikkei Average gained more than 300 points in the Tuesday morning session from the previous close.
It was the first time since April the index recovered the 30,000 threshold in intraday trading.
Analysts say investors are buying on optimism that Japan's coronavirus infections may have peaked.
Some also expect Prime Minister Suga Yoshihide's successor could come out with measures to spur the economy.