"After travelling more than 4,000 maritime miles an Iranian warship entered the Gulf of Aden to protect Iranian ships against pirates," reported the Iranian state radio, but gave no further details.
An unnamed official said the gulf was an international area and that Iran's armed forces would "carry out any decision made by their superiors".
The Iranian force joins ships from the EU, US, India, Russia, Malaysia and others which are already patrolling in the area.
China has also said it is considering sending a defensive force to the Gulf.
In October, Iran paid a ransom to free the crew of a captured merchant ship, and an Iranian-operated cargo ship carrying 36,000 tonnes of wheat was seized in November.
Pirate attacks are a regular occurrence in the Gulf of Aden, with many countries blaming the breakdown of law and order in Somalia.
World leaders have called for greater action to deal with the problem and last week, the UN approved a resolution allowing foreign troops to pursue pirates on land in Somalia.
