The agreement, which came after Grossi personally visited Tehran to meet with Iranian leaders, ended a months-long impasse over two locations thought to be from the early 2000s.
Iran had been permitting IAEA inspectors in to current nuclear sites agreed upon in the landmark 2015 nuclear deal with world powers, but had argued the other two sites dated from before the deal so there was no reason to grant access.
The IAEA in March identified the two sites as places where Iran possibly stored and/or used undeclared nuclear material or undertook nuclear-related activities without declaring them to international observers.
Grossi told the agency’s board of governors in Vienna that inspectors had already visited one site and would visit another later this month.
“I welcome the agreement between the agency and Iran, which I hope will reinforce cooperation and enhance mutual trust,” he said, according to a copy of his prepared remarks.
The ultimate goal of the nuclear deal is to prevent Iran from developing a nuclear bomb, which Iran insists it does not want to do.
But one reason the other countries involved — Germany, France, Britain, China and Russia — say that it's important to maintain the agreement is for the access IAEA inspectors continue to get to Iran's nuclear facilities.