Airlines were forced to delay and cancel some flights.
The German bomb was discovered buried underground near the transport hub's busy Terminal C, which handles flights to most major European destinations, the Dutch defence ministry said.
Schiphol cleared and closed Terminal C, parts of Terminal D and one landing strip, an airport spokesperson said.
A handful of European flights were cancelled while several dozen were delayed, she added.
The 500kg explosive, uncovered during construction work, would be taken to a safe location and dismantled, the ministry said.
Schiphol was a military airport during World War II. It was bombed both by the Germans at the start of the conflict and by Allied forces during the fighting.
Flights by airline KLM, which is part of Franco-Dutch group Air France KLM and uses Schiphol as its main hub, were affected, Schiphol's website showed.
Schiphol is Europe's fifth busiest airport and handled about 45-million passengers in 2010.
It is owned by the Dutch state, the cities of Amsterdam and Rotterdam, and French airports operator Aeroports de Paris . – Reuters