"The European Union will pay a very high price for this illegal and highly unethical action, which I have consistently warned them about," the US president writes on Truth Social.
The threats come after the EU fined iPhone maker Apple and Facebook owner Meta 500 million and 200 million euros respectively for violating the EU's Digital Markets Regulation.
On Thursday, it became clear that Google was also found to have violated the same regulation.
According to Trump, an investigation is now being launched into whether the EU is "robbing" US companies in this way. He expects the fines to be lifted and a "significant tariff" to be imposed on the EU.
