That compared with a 16.1% decline expected by economists in a Reuters poll and followed a 19.2% fall in July.
Imports dropped 20.8% in the year to August, compared with the median estimate of an 18.0% decline.
The trade balance came to a surplus of 248.3 billion yen ($2.36 billion), against the median estimate of a 37.5 billion yen deficit.
For the poll story:
To view the full tables, go to the MOF website at:
here Reporting by Daniel Leussink; Editing by Chang-Ran Kim