Exports fell to 15.5 billion francs ($16.9 billion) although the results are skewed by the fact there were two fewer working days in May this year than in 2014, the Federal Customs Administration (FCA) said in a monthly report.
Corrected for this variation, exports fell less severely by 0.8 percent.
All the same, the weakness of the euro hurt Swiss exports to the eurozone — Switzerland's biggest trading partner.
Without accounting for the calendar differences, exports to the eurozone tumbled 17 percent to 6.7 billion francs, while those to the EU as a whole dropped 16 percent to around eight billion francs.
When corrected for the difference in days, the watchmaking industry, with sales up 1.4 percent, was one of only three export sectors to see an increase in exports.
Sales to foreign countries of Swiss jewellery rose 24.6 percent on a corrected basis, while textiles, clothing and shoes advanced 4.7 percent.
The paper and graphics arts industry saw exports plunge 12.3 percent on a corrected basis, while those for the plastics sector fell 7.4 percent and the pharmaceutical and chemical sector by 3.8 percent.
Imports for the month declined by 17 percent on a nominal basis to 12 billion francs, leaving Switzerland with a trade surplus for May of 3.4 billion francs.
Part of the reason for the decline in imports is that their cost from the eurozone is lower because of the weak euro.
In the first five months of the year, Switzerland has built up a cumulative trade surplus of 14.2 billion francs, 1.8 billion francs more than in the same period in 2014.