TikTok is under investigation by The Irish Data Protection Commission (DPC) - its lead regulator in the EU - over two privacy-related issues.
The watchdog is looking into its processing of children's personal data, and whether TikTok is in line with EU laws about transferring personal data to other countries, such as China.
TikTok said privacy was "our highest priority".
The Irish DPC said it was specifically looking into GDPR-related issues.
These are the EU privacy laws which can potentially lead to enormous fines of up to 4% of a company's global turnover.
It said the first inquiry would examine "the processing of personal data... for users under age 18, and age verification measures for persons under 13". It will also look into how transparent TikTok has been about how it processes such data.