Israel has itself been condemned by the New York-based group and other campaigners for its use of cluster bombs during the 34-day conflict.
Human Rights Watch said it had been told by Israeli police of more than 100 documented cases of cluster rocket strikes.
Hizbullah made no immediate comment.
"We are disturbed to discover that not only Israel but also Hizbullah used cluster munitions in their recent conflict, at a time when many countries are turning away from this kind of weapon precisely because of its impact on civilians," said Steve Goose of Human Right Watch in the report.
Cluster bombs burst into bomblets that spread out over a wide area near the ground. Many bombs do not immediately explode, becoming land mines that can kill or maim humans well after the conflict.
770 sites hit by cluster bombs in Lebanon
The use of such bombs near civilian populations is forbidden by international humanitarian law.
Israel denies using the weapons illegally and has accused Hizbullah of firing rockets in Israel from towns and villages, using civilians as human shields.
A recent report by land mine activists identified 770 sites hit by cluster bombs in southern Lebanon before the ceasefire began on Aug. 14.
Human Rights Watch said the cluster munitions used by Hizbullah appeared to be Chinese-made 122 mm rockets, each one carrying 39 submunitions capable of shooting out hundreds of ball bearings.
Nearly 1,200 people in Lebanon, mostly civilians, and 157 Israelis, mostly soldiers, were killed during the fighting.