PIA flight PK-586, which was flying from Karachi to Bahawalpur in Punjab province, turned back to the port city after an air hostess alerted the captain about the passenger's threat, TV news channels reported.
The passenger who made the alleged threat was arrested by commandos of the Airport Security Force at the Jinnah International Airport.
The aircraft was searched by security forces after being evacuated.
Following his arrest, the passenger claimed he had made the alleged threat after a quarrel with the air hostess and that he had no intention of hijacking the aircraft.
The number of passengers on the flight could not immediately be ascertained.
Pakistan's aviation industry has been hit by a string of problems in recent years.
A total of 127 people were killed when a Boeing 737 of the private Bhoja Air crashed near Islamabad on April 20.
This was the second major air disaster in the vicinity of the Pakistani capital since July 28, 2010, when an Air Blue airliner slammed into the Margalla Hills in cloudy weather, killing all 152 people on board.
State-run PIA, which is the country's largest airline, is grappling with a funds crunch and an ageing fleet.
Commandos surrounded the aircraft as soon as it returned back to Karachi and after a while entered the plane and safely evacuated the passengers.
According to officials soon after the flight had taken off from Jinnah International Airport at 13:09, a passenger on board the plane told the cabin crew that he intended to hijack the plane.
"The cabin crew immediately informed the captain who spoke to the control tower to inform them that he wants to bring the plane back to Karachi.
"The plane, which had over 50 passengers on board, landed back in Karachi at 13:40 and the passenger was taken into custody by the ASF for interrogation," officials said.
However, they declined to provide identity of the arrested passenger.