European football's governing body said it would look at whether PSG had violated its Financial Fair Play (FFP) rules.
“The investigation will focus on the compliance of the club with the break-even requirement, particularly in light of its recent transfer activity,” UEFA said in a statement.
PSG signed Brazilian striker Neymar for a world-record 222 million euros on August 3rd and completed a loan move for Kylian Mbappe on Thursday in a deal that includes an option to buy the French teenager for 180 million euros.
“In the coming months, the Investigatory Chamber of the UEFA Club Financial Control Body will regularly meet in order to carefully evaluate all documentation pertaining to this case,” the statement added.
“UEFA considers Financial Fair Play to be a crucial governance mechanism which aims to ensure the financial sustainability of European club football.”
PSG said they were “surprised” by UEFA's decision.
“The club is surprised by such a step as it has permanently kept UEFA's Financial Fair Play teams informed of the impact of all player transactions this summer while nothing obliged it to do so,” read a statement on the PSG website.
“The club is very confident in its ability to demonstrate it will perfectly respect the Financial Fair-Play rules in accordance with the financial year 2017/18.”
Mbappe's move to PSG has left many wondering how the Qatar-owned club can avoid sanctions from UEFA considering the investment already made on Neymar.
According to media reports, the only clause required to be met to make the club's option to buy Mbappe an obligatory one is for PSG to stay in the top flight this season.
UEFA will have to consider whether Mbappe's transfer fee is factored into PSG's accounts for the current season, or whether it will carry over to the following campaign.
The FFP system was first approved by UEFA in 2010 to try and combat soaring debts in the game.
From 2013 to 2015 clubs could only post a net total loss of 45 million euros, which was reduced to 30 million for the next three years, running until 2018.
Non-compliance with the rules can result in a series of disciplinary measures, ranging from a warning to the deduction of points, and even exclusion from European competition.
PSG have already incurred sanctions for a previous violation and were slapped with a 60-million-euro fine, 40 million of which was suspended, in 2014, and saw their squad for the Champions League cut from 25 to 21 players.
READ ALSO: PSG sell 10,000 Neymar shirts on first day