Friday's announcement from European football's governing body said it would look at whether PSG had violated the regulations, which are created to prevent football clubs spending more than they earn.
At that time, he is expected to become the world's second most expensive player, with the deal reported to be as much as €180 million.
UEFA will not give any further comments on this matter as the investigation is now ongoing.
The Uefa statement said the investigatory chamber " will regularly meet in order to carefully evaluate all documentation pertaining to this case".
Uefa has announced a formal investigation into the multi-million pound transfer window spending of Paris Saint- Germain, and whether it has complied with financial fair play (FFP) rules requiring clubs to break even.
"The Club is surprised by the fact that it has constantly kept the UEFA Financial Fair Play teams informed of the impact of all player operations carried out this summer, were obliged to do so". Some within Uefa are thought to have wanted to ban PSG from the Champions League then, but they were not a majority. "I am very serious", he said on Friday.
Barcelona were also linked with late move for Mbappe but that deal seemingly didn't come to fruition.
No explanation was given as to why Mbappe has been initially signed on loan, but local media said it was an attempt to spread the payment to ensure PSG do not fall foul of FFP rules aimed at stopping clubs with rich owners spending their way to success.
"Among players of his age, he is without doubt the most promising in the world due to his huge technical, physical and mental qualities", PSG president Nasser Al-Khelaifi said.
Reports in Spain had suggested that both Barcelona and Real Madrid - whom were long considered the frontrunners to sign Mbappe - had complained to UEFA about PSG's conduct this summer.
Neymar's world-record arrival at PSG was followed up by the acquisition of Kylian Mbappe. They thus have made sure the transfer fee for Mbappe will only be taken into account next year.
"The Club has demonstrated in recent years that it has the capacity to grow its income exponentially and anticipates an additional 20 to 40% increase on all the levers it has (International Revenue, Sponsoring, Merchandising, Ticketing, TV rights, income related to sports results, summer and winter tours)".
"The loan agreement also includes a call option for Paris Saint-Germain which, when exercised, will bind the player to Paris Saint-Germain until June 30, 2022", a statement on their official website read.