The fall was smaller than a 10.9% drop expected by economists but marked the longest run of declines in exports since a 14-month stretch from October 2015 to November 2016.
"It's worrying that exports to Asia and especially China are falling," said Atsushi Takeda, chief economist at Itochu Economic Research Institute.
"There's no doubt that the Chinese economy is slowing down as the impact of the third round of U.S. tariffs is starting to be felt."
Exports in volume terms, which strips away the exchange rate impact, fell 6.0% year-on-year in August, more than reversing July's increase, which was the first in nine months.
Fears of a global slump have persisted in recent weeks as uncertainty over U.S. trade policy clouds the outlook for growth in the United States and export-reliant economies such as Germany, Japan and China.
"The high growth rate of the U.S. economy that was seen until now could come down," said Takeshi Minami, chief economist at Norinchukin Research Institute.
"There's a possibility exports to the United States will stagnate on the back of that."
The nine-month slump in Japan's exports follows revised data that showed the economy grew at a slower-than-expected pace in April-June as business spending was downgraded from initial estimates.
The latest data comes a day before BOJ policymakers on Thursday decide whether to expand an already massive stimulus programme to shield the economy from a possible downturn and protect its growth drivers.
Increasing signs of slowing global demand have made Japanese central bankers less confident about an early pickup in global growth and more open to debating policy easing.
Though service-sector activity remains resilient in Japan, analysts are worried domestic consumption could take a hit after the government raises the sales tax to 10% next month.
"There are hopes exports will rebound as domestic demand is expected to drop to some extent following the sales tax hike," said Itochu's Takeda.
Imports in August fell 12.0% from a year earlier, the figures showed, the sharpest drop since a 16.3% decline in October 2016, according to Refinitiv data.
Asian demand for cyclical tech products remained especially weak in August, he added, in a sign that a bounce in exports may not be imminent.
Separate polls this month showed business confidence among Japanese manufacturers hit the weakest level in 6-1/2 years, while most companies have put capital investment plans on hold amid fears about the economic outlook.
By region, exports to China, Japan's biggest trading partner, shrank 12.1% year-on-year in August, down for the sixth month and driven by declines in shipments of semiconductor manufacturing equipments and car parts.
Shipments to Asia, which account for more than half of Japan's overall exports, dropped 10.9% in the year to August.
Japan's exports to the United States slipped 4.4% in the year to August, their first fall since September last year and driven by declines in exports of 3000cc cars and car parts.
Imports from the United States dipped 9.2%, causing Japan's trade surplus with the world's biggest economy to rise 3.8% from a year earlier to 472.0 billion yen, the data showed. Washington and Tokyo have agreed to an initial trade deal that is expected to be signed in New York later this month, but full details of the agreement have not been disclosed.
One critical point for Japan left unanswered is whether threatened U.S. national security tariffs will be imposed on Japanese vehicles and auto parts, which together account for roughly 30% of exports to the United States.
(Reporting by Daniel Leussink; Editing by Sam Holmes) ((email@example.com; Twitter: @danielleussink; +81-3-6441-1825;))