BEIJING—A string of Chinese internet lenders have shut their doors in recent weeks, stranding investors as the economy slows and regulators tighten controls over an unruly side of the fintech sector.

Across China, more than 200 internet-based fund managers since late June have either shut down, closed parts of their operations or are reeling from cash crunches, missing executives and other problems, according to industry tracker Wangdaizhijia.

The...