Only 35.5GW of new wind farms were installed in 2013, nearly 10GW less than the year before.
The fall in installations was largely down to the "precipitous drop" in new projects in the US due to the expected halting of the production tax credit subsidy regime at the end of 2012, the industry body said.
GWEC characterised 2013 as a "difficult year for the industry" with only 12.5% cumulative growth, but said that it expected 2014 to be "much brighter".
Global cumulative capacity hit 318GW, an increase of nearly 200GW over the last five years.
GWEC secretary general Steve Sawyer welcomed strong installation figures from China that showed the country adding 16.1GW, almost half of the global new capacity.
"China is a growth market again, which is good news for the industry. The government's commitment to wind power has been reinforced once again by raising the official target for 2020 to 200GW, and the industry has responded," commented Sawyer.
Brazil led the charge in Latin America, increasing its capacity by more than a third, with the installation of 948MW of projects.
Canada outstripped the US, adding 1.6GW in 2013, compared with 1.1GW in the US. Although projects under construction hit an all-time high in the US, with work having started on 12GW of developments.
Europe as a whole installed 12GW, 8% less than in 2012, with a large proportion focused in Germany and the UK.