The important border post is crucial for bringing provisions and supplies to those in need in the Gaza Strip. But the crossing was closed following the US and Israeli attack on Iran on February 28.
Israel said the reason was "security measures" and it simultaneously closed all other border crossings.
Kerem Shalom and Rafah in southern Gaza reopened later, but Zikim only opened on Monday.
"The opening helps meet the critical need to get supplies directly to northern Gaza," UN spokesman Stéphane Dujarric said.
Deliveries from, among others, the UN World Food Programme (WFP) and UNICEF were able to resume on Monday.
