"The criticisms are focused on features and settings that were in place three years ago, and that we made changes to well before the investigation even began, such as setting all under 16 accounts to private by default,"
The fine was issued by Ireland's Data Protection Commission (DPC) under the EU's General Data Protection Regulation (GDPR) privacy law.
Despite the fine being in the hundreds of millions, it is actually smaller than other penalties seen in recent months - such as the €1.2bn (£1bn) fine Meta was given by the regulator in May for mishandling people's data when transferring it between Europe and the United States.
It is however substantially larger than the £12.7m fine TikTok was issued by the UK data watchdog in April for allowing children aged under 13 to use the platform in 2020.
The fine issued by the DPC specifically refers to 2020, and TikTok took several actions in the years following to make it more compliant.
This included it becoming one of the first social media sites to make accounts for 13 to 15-year-olds private by default in January 2021.
It will also introduce a change this month which will mean all 16 and 17-year-olds signing up to the platform will have their account set to private by default.