Pope Benedict XVI hailed the ban and joined calls for all countries to sign up to the deal that also obliges those that have ratified it to destroy their stockpiles of the bombs.
China, Russia, the United States and Israel are among those that have rejected the 2008 Convention on Cluster Munitions, ratified by around 37 countries and signed by 107.
The big military powers that have not signed up are thought to hoard and manufacture the bulk of the munitions, although the data is secret.
Campaigners including the Red Cross hope the moral weight of the treaty will persuade them to sign up.
Sunday's milestone "stigmatises the use of cluster munitions," said International Committee of the Red Cross chief Jakob Kellenberger.
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"We hope that the entry into force will also affect the practice of states that have not yet adhered to the treaty," he said.
The United States alone accounts for cluster bombs or shells containing around 800 million bomblets, according to the Clus