The central bank reported 6.5pc growth in the third quarter, marking the lowest rate of expansion since the financial crisis in 2009 and falling marginally short of the 6.6pc figure that was widely expected.
NBS spokesman Mao Shengyong said China's economic growth remained generally "faced with an extremely complex environment overseas and the daunting task of reform and development at home".
After another big decline in Chinese stocks on Thursday, policymakers sought to soothe markets, with central bank governor Yi Gang saying the recent stock market fluctuations are largely driven by investor sentiment and equity valuations are not in line with economic fundamentals.
Analysts had expected growth of 1.6 percent on a quarterly basis.
"The 6.5 percent figure is definitely below our consensus expectations".More news: 'There's no God; no one directs our fate'
Before the data release, economists had expected China's full-year growth to come in at 6.6 percent this year - comfortably meeting the government's 6.5 percent target - and 6.3 percent next year.
However, some now warn growth could slow even more dramatically next year.
China's economy grew at a slower quarterly pace than expected, expanding 6.5 per cent in the three months ended September, as the country's trade war with the U.S. exacted a toll on exporters and manufacturers.
China's exports unexpectedly kicked into higher gear in September, largely as firms front-loaded shipments to dodge stiffer USA duties.
The service sector gained 7.7 percent year-on-year in the January-September period, picking up from a 7.6-percent increase in the first half, and outpacing 3.4 percent in primary industry and 5.8 percent in secondary industry.More news: Amritsar: Many feared dead in train accident
Factory output grew 5.8 per cent in September from a year earlier, down from 6.1 per cent in August, while retail sales rose 9.2 per cent year on year last month, up from 9 per cent in the previous month, and accelerating for the second month in a row.
The central bank has cut reserve requirements for lenders four times this year, with the latest cut taking effect on October 15, injecting more liquidity to stimulate bank lending.
China is locked in a trade war with the United States, with Washington threatening to escalate the confrontation after imposing tariffs on 0 billion of Chinese goods.
The slowdown is likely associated with the crackdown on riskier lending and debt and the ongoing trade war with the US.More news: Samsung Launches Galaxy Book 2; Features Qualcomm Snapdragon 850 And Gigabit LTE