The world's second largest economy grew by 4.9%, year-on-year, in the three months between June and September, when in the previous quarter it had grown by 7.9%, according to Chinese government data.
Factory output, retail sales and investment in construction and other fixed assets weakened.
Growth faces pressure from a vast regulatory campaign as Beijing tries to make China's economy more energy efficient and reduce the galloping increase in debt, public and private, to avoid financial problems.
The manufacturing sector has been hurt by shortages of processor 'chips' and other components due to the covid-19 pandemic.
Compared to the previous quarter, production in the July-September period was virtually stagnant, expanding by only 0.21TP2Q.
This was lower than the April to June period of 1.2%, one of the weakest in the last ten years.
"Growth will slow down further," Louis Kuijs of the Oxford Economics consultancy said in a report.
The analyst argued that the "weak growth numbers" should lead Beijing in the coming months to ease controls on credit and boost economic activity by encouraging infrastructure construction.
Construction, which supports millions of jobs in China, has slowed since regulators increased controls last year on corporate debt.
One of the largest construction companies, Evergrande Group, is struggling to meet interest payments on debt securities amounting to billions of dollars.
This has stoked fears that the company's collapse could spread to other construction companies, although economists have said the threat to global financial markets is small.
The manufacturing sector has been hurt by the reduction in power supply, imposed by some provinces, to avoid exceeding official energy efficiency targets.
Factory output slowed in September by expanding by only 0.05% compared to the previous month.
This was below the 7.3% growth in the first nine months of the year.
Private sector analysts have lowered the growth outlook for the Chinese economy, for this year, although they still expect an increase of about 8%, which would be among the strongest in the world.
The Chinese Communist Party's official target is "above 6%," which leaves room for Beijing to maintain its regulatory campaign.
"The near-term outlook for China's economy in the fourth quarter remains difficult due to the impact of continued winter energy shortages and the ongoing slowdown in the real estate sector," Rajiv Biswas of consulting firm IHS Market said in a report.
"The real estate sector continues to be hit by uncertainties, related to Evergrande's debt problems and fears of contagion in other real estate companies."
This year's numbers are exaggerated by the base comparison with 2020, when factories and retailers were closed due to the containment measures.
In the first quarter of 2021, the Chinese economy grew a record 18.3%.
In September, retail sales growth slowed by 4.4% year-over-year.
Investments in real estate, factories, housing and other fixed assets increased by 0.17% in September, after increasing by 7.3% in the first nine months.
Source: Lusa