The firearms were found packed in individual boxes aboard a Dutch-registered truck that arrived on a ferry in the port city.
Although customs rules were not broken, the Turkish driver did not have licences for the weapons, police said.
Controls at Italian ports have been stepped up in the wake of the 13 November Paris terror attacks.
EU interior ministers meeting in Brussels last week agreed to tighten security and share more information about the gun trade to prevent trafficking of illegal weapons.
It came after attackers with automatic rifles and suicide belts killed 130 people at a number of sites in Paris. Some of the attackers had lived in Belgium.
The 781 weapons seized in Trieste on Tuesday were pump-action Winchester SXP shotguns, according to a police statement (in Italian).
Prosecutors are investigating possible charges.
The Turkish manufacturer of the weapons was cited for breaching national security regulations, Ansa news agency reported.
Belgium has historically been a European hot spot for the arms trade - legal and illegal - due to its relatively liberal domestic gun legislation.
Even though Belgian legislation is now stricter and more solid, the country has retained this reputation.
Italian police in the northern city of Trieste have seized nearly 800 shotguns en route from Turkey to Belgium.