Prosecutors in the southwestern city of Stuttgart, a car industry bastion, said in a statement they had opened a probe against "persons unknown working for the car maker and a US subsidiary".
The investigation into "suspicion of fraud and false advertising" stems from "possible manipulation of exhaust treatment in diesel vehicles from Porsche AG".
Porsche spokesman Christian Weiss told AFP that the company "takes the prosecutors' investigation very seriously" and would "do the utmost to clear up the issue comprehensively and as quickly as possible".
Volkswagen, the world's largest carmaker, admitted in September 2015 to using so-called "defeat device" software to cheat regulatory nitrogen oxides emissions tests in some 11 million cars worldwide.
The devices allowed the cars to spew up to 40 times the permissible limits of nitrogen oxide during normal driving, but this was hidden during emissions testing.
Present Volkswagen chief executive Matthias Mueller led Porsche AG between 2010 and 2015, taking charge at the parent company after his predecessor Martin Winterkorn stepped down over the "dieselgate" revelations.
Stuttgart prosecutors said in May they were investigating Mueller over market manipulation, suspecting he failed to share information about the diesel cheating scandal quickly enough with shareholders as a director of Porsche SE.
Porsche SE, separate from car maker Porsche AG, is a company owned by descendants of VW Beetle inventor Ferdinand Porsche that holds a controlling stake in Volkswagen.
VW faces an array of legal challenges in Germany and worldwide relating to defeat device software, installed mainly in own-brand vehicles but also in cars made by Audi, Porsche, Skoda and Seat, among its stable of 12 brands.
In the United States, VW settled criminal and civil charges against it with $4.3 billion in fines.
The carmaker has agreed total payouts of some $23 billion, especially in compensation to around 600,000 car owners.
Meanwhile, prosecutors in Munich, southern Germany, last week arrested a former Audi executive wanted in the US in connection with the scandal.
The Italian suspect led a team of engineers responsible for designing emissions control systems for diesel vehicles in the United States between 2006 and late 2015.