The August rate was up 0.2 percent from the previous month as the country continued to be dogged by sluggish economic conditions, which may have adversely affected corporate sentiments on increasing payrolls.
The number of the jobless in August totaled 2.72 million, up 230,000 from the same month of last year, Kyodo news service reported, adding the 4.2 percent rate came in higher that the market forecast of 4.1 percent.
"The result added to the sign that the Japanese economy has been weakening," Japanese Economic and Fiscal Policy Minister Kaoru Yosano said to Kyodo.
The minister urged for speedy government action to resolve the problem.
There were 64.05 million people holding jobs in August, down 410,000 from the same month of last year. It was the seventh consecutive month of fall in the number of employed, the report said.