King Tomislav (died 928) was a ruler of Croatia in the Middle Ages. He reigned from 910 until 928, first as Duke (dux Croatorum) of Dalmatian Croatia in 910–925, and then became first King (rex Croatorum) of the Croatian Kingdom in 925–928.

He was probably the son of Muncimir, Duke of Dalmatian Croatia. Tomislav was one of the most prominent members of the House of Trpimirović. He united the Croats of Dalmatia and Pannonia into a single Kingdom in 925. He ruled over the territory of today's Croatia and Bosnia, rounding off his state from the Adriatic Sea to the Drava River, and from the Raša River in Istria to the Drina River

Tomislav defeated the Magyar mounted invasions of the Arpads in battle and forced them across the Drava River. Tomislav annexed a part of Pannonian Croatia to his Croatian Dalmatia. This included the area between the rivers Drava, Sava and Kupa, so his Duchy bordered with Bulgaria for a period of time. This was the first time that the two Croatian Realms were united, and all Croats were in one state.

The Duke had to face renewed threats from the Bulgarians under Tsar Simeon I who had already conquered the Serbs. In 923, the Patriarch of Constantinople and the Byzantine Emperor offered to deal with Simeon's threat if Pope John X would accept a rejoining of the divided Sees of Rome and Constantinople. The Pope also demanded that the Patriarch give him the sovereignty over the Dalmatian Byzantine Cities. After this was done, the Byzantine Emperor gave Duke Tomislav the coastal Cities under his Governency: the Byzantine Province of Dalmatia (Zadar, Split, Trogir...). In 921-924, the Bulgarian leader Simeon struck through Rascia, enslaving the people, which made many Serbs under the dethroned Prince Zaharije Pribislavljević of the House of Vlastimirović flee and seek shelter in Tomislav's Realm.

At the peak of his reign, according to Byzantine emperor Constantine VII Porphyrogenitos' De Administrando Imperio, written around 950, Tomislav could've raised a vast military force composed out of 100,000 infantrymen and 60,000 horsemen and a sizable fleet of 80 large ships and 100 smaller vessels.[3] These figures are largely disputed due to the historical period, historians argue that the numbers are a clear overexaggeration and an overemphasis that should be interpreted differently.