It is believed that the name of the town and municipality of Kostajnica originates from the old Slavic word for sweet chestnut (kostanj).

In the middle there is a chestnut with a leaf that represent the origin of the name of this municipality and of the rare plant that grows in this area.

In the upper middle of the coat of arms the number 1258 is written, which represents the year when name Kostajnica was first recorded.

The final name of the town and municipality is only Kostajnica, according to the decision of the Constitutional Court of Republika Srpska.

The settlements that make up the territory of this municipality are: Gornja Slabinja, Grdanovac, Gumnjani, Kalenderi, Kostajnica, Mrakodol, Mraovo Polje, Petrinja, Pobrđani, Podoška, Tavija and Zovik.

According to the found remains and preserved documents, it is known that in the era of Ancient Rome, there was trade across the Una River in the territory of the present municipality of Kostajnica.

During the French rule, a wooden bridge was built between the two "Kostajnicas" (the two towns that are situated on both banks of the Una river) to facilitate the flow of goods and people.

That was at the time the only bridge on the Una river that served to transport goods from the Middle East to Europe and vice versa.

This is evidenced by the legendary Balj Company (named after the hill above Kostajnica), which was made up of fighters mostly recruited from the area.

In addition to the fallen fighters, as many as 1,848 inhabitants of the municipality lost their lives as victims of the fascist terror war.

Una-Sana  Central Bosnia Posavina  Herzegovina-Neretva Tuzla  West Herzegovina Zenica-Doboj  Sarajevo Bosnian Podrinje Canton 10