However, sales to the rest of the United Kingdom expanded by 6% to £13.8bn.
Great Britain sales have become relatively more important to Northern Ireland businesses in recent years.
The Republic of Ireland continues to be the single, biggest export market with sales of £3.4bn.
The figures come from the Northern Ireland Broad Economy Sales and Exports Statistics report, an experimental data series compiled by government statisticians.
It shows that manufacturing continues to dominate Northern Ireland exports, accounting for 60% of sales.
However, manufacturing exports fell by almost 4% to £5.5bn in 2015.
Exports to all major markets fell: down by 4% to the Republic of Ireland and by 3% to the USA.
The number of businesses selling outside Northern Ireland fell marginally, down by 67 to 11,467.
A different, more recent, set of figures from HM Revenue and Customs suggests that Northern Ireland's manufacturing export performance is improving at a better rate than any other part of the UK.
HMRC data for the 12 months to September 2016 showed a 6% rise in the value of exports to £7.4bn.
Businesses have been helped by a slump in the pound, which makes their goods more competitively priced in the Eurozone and the United States.
The value of goods and services exported by Northern Ireland companies fell by 5% in 2015 to £9.1bn.