European football's governing body introduced FFP 'break-even' rules in 2013, requiring clubs to balance their spending with their revenue.
PSG more than doubled the world record transfer fee when they spent 222m euros (£200m) to sign Neymar last month.
They have signed Kylian Mbappe on loan, and could pay £165.7m for him in 2018.
PSG have been owned by the Gulf state of Qatar, via its Qatar Sports Investments fund, since 2011.
The French club made made a profit of 10m euros (£9.2m) on revenue of 520.9m euros (£389.6m at the time of conversion) in the 2015-16 financial year, according to the most recent figures published by Deloitte.
The six-time French champions have failed to meet FFP rules before - receiving a fine, a spending cap and only being allowed to register 21 players for the Champions League in 2014.
Uefa's FFP regulations were approved in 2010 and the first assessments began the following year.
The 'break-even' element states clubs can spend 30m euros more than they earn over a three-year period.
A Uefa statement said the investigation into PSG would "focus on the compliance of the club with the break-even requirement, particularly in light of its recent transfer activity".
It added: "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.
"Uefa considers financial fair play to be a crucial governance mechanism which aims to ensure the financial sustainability of European club football."
Uefa has opened a formal investigation into Paris St-Germain as part of its "ongoing monitoring" of clubs under financial fair play (FFP) regulations.