The value of Welsh exports fell by £827 million over the last 12 months up to March, according to new statistics from the Welsh Government.
The figures show a fall in the value of iron and steel exports and also in petroleum products.
There was a fall in exports to EU countries of £740m and a fall in exports to non-EU countries of £87m.
Exports to the EU accounted for 39.8% of Welsh exports.
The Welsh Conservatives have criticised the Labour-led Welsh Government for not doing enough to support businesses.
A spokesperson said: “If this downward spiral continues then Wales’ standing on the international market will be significantly diminished.
“There is so much more that Welsh Labour government ministers could be doing to support Welsh businesses in their efforts to tap into overseas markets and they must not allow these figures to drop further.
“Labour has its hand on a number of economic levers that could grow the Welsh economy and exports play a significant part in that.
“Always anxious to trumpet any increase in exports, it is sadly inevitable that today’s development will be greeted with near-silence.
“Our export market remains fragile. The only way to change this is by developing a long-term sustainable plan, working side-by-side with businesses, to give them the confidence to invest, create jobs and boost their output.
“Such a plan is woefully absent from the Labour Government’s economic strategy. They must do more to work with the UK Government to ensure that Wales reaps the benefits from what is now the fastest growing economy in all of the G8 economies.”
However, Economy and Infrastructure Secretary Ken Skates AM hit back at the criticism.
He said: “It is widely recognised that the fall in commodity prices, and petroleum products in particular, has played a major part in the fall in the value of goods exported from Wales.
“The facts are that Welsh exports in the first quarter of 2016 were actually higher in value than in the previous quarter, up £83m or 2.9%, and this bucked the UK trend where there was an overall fall in exports of 2.7%. Since 1999 there has been an 89% increase in Welsh exports while in the same period, UK exports increased by only 69%
“Supporting companies to export is one of our key economic development priorities and we have a full range of support to help both existing and aspiring exporters across all sectors in Wales and wherever they are on their export journey. This could range from helping to build capability to connecting with potential customers and getting to overseas markets.
“In 2015/16, Welsh companies secured more than £72m in export business as a direct result of this Welsh Government support. This is a £20m increase on the previous year and shows that our approach is working.”