No other candidate was nominated by any of the union's branches by the deadline of 5pm on 23 October and the PCS announced that there would be no membership ballot.
Serwotka will now serve for another five years.
Elected in 2000, Serwotka has been one of the most vocal and consistent opponents of spending cuts imposed by Labour and Tory governments.
PCS national president Janice Godrich said: "I'm delighted that Mark has been elected unopposed. It is a clear endorsement from members that he has the confidence and full support from our union.”
Serwotka said: "It is an absolute honour to have been re-elected and I look forward to working with our members and others to continue fighting this government's cruel, damaging and unnecessary obsession with austerity.”