The investigation concerns reports that X's Grok AI chatbot was used to create deepfake sexual images.
Allegations include the generation of undressed images, potentially amounting to intimate image abuse or pornography, and sexualised images of children.
Ofcom contacted X on 5 January, setting a deadline of 9 January for the company to explain its user protection measures.
Following X's response, Ofcom decided to open the investigation to determine if the platform has breached its obligations under the Online Safety Act.