"This is the first capture carried out in international waters to implement international conventions… to detain and inspect a ship not flying a national flag," the police said in a statement.
The 30-metre long vessel was referred to by police as a "mother ship" used by people smugglers to bring migrants to Italian waters then put them on smaller vessels to reach the coastline.
It was spotted 200 kilometres (120 miles) south of Sicily by a Romanian patrol boat taking part in the European Union's Frontex border guard service.
Italian police have arrested all 15 crew members and detained the ship itself.
The migrants included 85 men, 50 women and 84 minors, and all said they were refugees from Syria.