Anders Fogh Rasmussen said NATO would intervene if a future peace treaty was broken or if the two sides needed assistance.
Until then, NATO would "not (become) involved in the Mideast peace process and is not seeking a role in it," Rasmussen said.
The NATO leader spoke at an annual security conference in the Israeli city of Herzliya, north of Tel Aviv.
A spokesman for Israeli Prime Minister Benjamin Netanyahu said he and Rasmussen discussed expanding cooperation between Israel and NATO. The spokesman would not say if Israel would accept Rasmussen's offer.
Netanyahu insists Israel must have strict security guarantees in a peace deal.
Palestinian President Mahmoud Abbas has said he would agree to have international peacekeepers deployed in a future Palestinian state for a limited time.
Also Wednesday, Israeli planes attacked three targets in Gaza before dawn. Palestinian officials said eight people were slightly wounded. The Israeli military said the airstrikes were in retaliation for Palestinian rocket and mortar fire from Gaza on Tuesday.