Shankar has been entrusted the additional charge with immediate effect, PNB said in a statement.
A notification to this effect has been issued by Department of Financial Services, it added.
The post of full time Managing Director and CEO has been vacant since November, when the then Chairman and Managing Director K R Kamath failed to get extension from the government.
As per the government rules, the CMD and Directors are appointed in public sector entities for a period of five years or up to the age of 60, whichever is earlier.
The country's second largest public sector bank by assets had reported net profit of Rs 755.41 crore for the October- December quarter of the 2013-14 financial year.