The names of the settlements from that time are not known, but it is certain that there were many along the roads, roads and paths of those settlements. Traces of a Roman aqueduct and some buildings were found in Kostajnica, and not far from Kostajnica, in the village of Mrakodol (Briševci), there are archeological finds about a Roman iron smelter.
Over time, the settled Slavic tribes merged with the found natives, received many cultural heritage from them, and imposed their language and name on the natives. There is very little preserved material about Kostajnica from the Middle Ages. The name Kostajnica is mentioned for the first time in the state documents in 1258 in the decree of King Bela IV which resolves the dispute over the land "Kostajnica" between the order of the Templars from Dubica and Hetinka, a citizen of Kostajnica. It is assumed that the name Kostajnica originated from the word "chestnut" because the area on both sides of the Una river is rich in chestnuts. Since then, Kostajnica has changed many rulers. Count Arlandi was the first owner of Kostajnica, and then Count of Celje. After the death of Martin Frankopan, Kostajnica was ceded to the Duke of Krupa, Ivan Revanud. King Matthias donated a special donation to many Serbian villages and towns to the Serbian despot Vuk Grgurević Branković, and Kostajnica is mentioned among them. After Vuk's death, Kostajnica was in the possession of the despot Đurađ Branković. Thus, Kostajnica changed many rulers in a very short time.
With the victory over the Turks near Dubica in 1513, Kostajnica fell into the possession of Ban Petar Berislavić. Already in the first half of the 16th century, the town and the fortress were the property of the Zrinjski family - a gift from King Ferdinand. During the period of Zrinjski family, the fortress was specially fortified and upgraded. According to these lords, the old Kostajnica fortress is called "Zrinjski town". In 1556, the Bosnian bey Malkoč besieged Kostajnica, which he conquered thanks to the betrayal of Captain Lustthaler, and since then Kostajnica has remained under Turkish rule. The famous travel writer Evliya Çelebi came to Kostajnica in 1660 and wrote, among other things also:
"This city was built by Zrinjski with the help of the Hungarian King Ferdinand. Its nahiyah has 40 villages…
The city's population is made up of soldiers who fight with Croats over day and night, losing each other's heads…
Kostajnica is a huge town, built of hard material. Its shape is pentagonal, and it lies on the banks of the Una River, in the territory of Banja Luka. "
With the Peace of Požarevac in 1718, Austria, as a Venetian ally in the war with the Turks, gained Banija region and the area on the right bank of the river Una, in depth to the Kozara mountain.
In 1739, with the peace in Belgrade, all lands were returned to the Turks, and the Una River again became the border between Austria and Turkey. As an integral part of the Illyrian provinces in 1809, the area of Kostajnica came under the rule of Napoleon, and remained until 1814. During the French rule, a wooden bridge was built between the two Kostajnica towns, which facilitated traffic. The only bridge on the Una at that time was used for the transport of goods that went from the Middle East to Europe and vice versa. Customs for these goods is being opened in Kostajnica. However, after a short French rule, Kostajnica fell under Turkish rule again in 1814.
During the periods of Turkish rule, the population of this area raised numerous rebelions. The most famous is Pecija's rebelion in 1858, which was raised by Petar Popović Pecija and Petar Garača. Petar Popović, aka Pecija (1826—1875) was born to father Petar and mother Ilinka in the village of Bušević in Krupska. He spent most of his life in the town of Kostajnica. Petar learned to read and write on his own, and in addition to Serbian, he also spoke Turkish language. While not recognizing the Ottoman rule, Petar went to the guerilla at the age of 22 and from then until the end of his life he actively fought against the Turks. In the autumn of 1858, together with other rebels (hajduk) harambasha Petar Garača from Strigova, raised a great rebelion in Knešpolje and built a moat known as "Pecija's moat" in front of Bosanska Kostajnica. The people also join the rebels, so a fierce battle between the Turks and the insurgents takes place on the trench. After a long and difficult battle, the Turks managed to force the insurgents across the Una River into Hrvatska Kostajnica, where most of them surrendered to the Austrian guards. Pecija and Garača with about 300 other insurgents did not surrender, but withdrew to Pastirevo. This rebelion is popularly known as the Kostajnica or Pecija uprise, and less often as the Knešpolje uprise. After the Kostajnica rebelion, the Turks captured Pecija twice. He was sentenced to death in Constantinople, with the verdict that he would be executed in his area because he "committed the most evil crimes there". On the way to his hometown, Pecija fled of the Turks near Kragujevac, where he stayed and got a guard position in the cannon foundry. He remained there until the new uprising in Bosnian Krajina in 1875. He joined the insurgents and already at the beginning of September took part in the battle near the village of Gašice, now Gašnice. On that day, Petar Pecija once again proved to be a capable leader of the rebels, overpowering the Turks and forcing them to flee. However, shortly afterwards, and according to folklore due to the betrayal of a certain Ilija Čolanović, the Turks surprised the insurgents and after a long and fierce battle defeated them, some of whom fled to Prosara, and about fifty of them, led by Pecija, remained to fight on the coast of Sava river. Chased by the Turkish force, Pecija and the insurgents managed to grab the boat and cross the river, but without shelter, exposed to Turkish gunfire, almost all perished. Only Pecija and two men survived, but Petar, restless and brave in spirit, instead of taking cover, went out into the clearing and shouted at the Turks: "Hey Turks, I want to let you know, you did not kill Pecija, he is still alive, and he will take revenge on you soon". At that moment, he was hit by a Turkish bullet and killed on the spot. It was September 10, 1875. There was a rumor among the people that he was not killed by a bullet but that the rifle was loaded with wheat. Comrades buried the dead Pecija with Kormanoš and other heroes in the village of Jablanac below Jasenovac, and ten years after Pecija's death, the bones were transferred and stored near the Moštanica monastery at the foot of Kozara, near Kozarska Dubica.
In 1862, Muslims who had previously converted to Islam came to Bosanska Kostajnica from the vicinity of Valjevo and Šabac, while many of them came from Užice. Here in Kostajnica, they formed a settlement that still bears the name of Uzice, after their old homeland.
During the Nevesinjska puška (Nevesinje rebelion) in 1875, this area offered resistance, and Petar Mrkonjić came to Kostajnica several times, whose organizational role in the fight against the Turkish rule was of great importance. On August 15, 1875, the villagers of Čitluk, Petrinja, Bačvani, Pobrdjani, Tavija and other surrounding villages attacked the Turkish garrisons in the village of Dvorište. Enthusiastic about the initial successes of the uprising, the insurgents under the command of Marko Bajalica and Marko Đenadija attacked Bosanska Kostajnica on August 19, but after the initial success they had to withdraw. In those battles with the Turks, Petar Garača was killed, whose grave is located in Gornja Slabinja in the hamlet of Alaginci, so the place itself is named after him - Garača's grave.
In 1878, Kostajnica fell under Austrian rule again and remained so until the end of the First World War. Initially, Austria introduced a military regime, and later replaced it with civilian rule. During Austria, Kostajnica was firstly a district, and later a branch of the Bosanski Novi district.
In the Kingdom of Yugoslavia, Kostajnica was a developed trade and craft center. The richest and most prominent trading families from that time were: Marin, Zec, Pavlović, Jandrić, Savić and Lukić. Service activities were developed and fully meet the needs of the population. This trend continues in the post-war period. The trade company "Vis" was developed with a total of 4 grocery stores and two shoe stores. The following services are represented by service activities: barber, tailor, butcher, carpenter, shoemaker, mechanic, etc. The largest company was Factory of agriculture machines, created from a blacksmith shop, which was founded in 1908 by brothers Meho and Rasim Ferhatović. They made their own designs of their well-known plows, harrows and drills, and won medals at fairs for them. Brothers August and Heinrich Vandispil from Switzerland had the best agricultural property in the village of Tavija. Impeccably landscaped orchards, gardens and fields were a model even for farmers from distant places.
During the Second World War, the Serb population of this area participated in the fight against fascism. The legendary "Baljska četa" (troop) was composed of soldiers, mostly recruited from the area of Kostajnica. In the fight against fascism, this area produced over 1,000 soldiers, of which 460 died, not waiting for the end of the war. In addition to the fallen soldiers, the victims of fascist terror were also the population of the municipality, where in memory of them after the war, two memorial ossuaries were built on Zecevo brdo for over 2600 of innocent civilians, mostly women and children of Serb nationality from this and other areas, killed by Ustashas. The Jasenovac concentration and death camp, Bajića jame (hitch) and Zečevo brdo (hill) are their execution sites. In addition to human casualties, the area of the municipality suffered enormous material destruction, and 108 households were completely destroyed. After the war, two partisan cemeteries were built in Kostajnica and Petrinja. This area produced legendary partisan soldiers and commanders, of whom some were declared as national heroes: Petar Borojević, Lazo Šteković, Petar Mećava, Branko Tubić and Mile Tubić, and no less famous are the legendary doctors Mladen Stojanović and Ranko Šipka who raised and organized an uprising in Balj forest.
After the Second World War, Bosanska Kostajnica existed as a municipality within the Bosanski Novi, and later Prijedor district. The new territorial division on December 31, 1962 abolished the municipality of Bosanska Kostajnica, and its territory was divided between the municipalities of Bosanska Dubica and Bosanski Novi. The municipality of Bosanski Novi included the following settlements: Bosanska Kostajnica, Petrinja, Gornja Slabinja, Podoška, Zovik, Pobrđani, Kalenderi, Mraovo Polje, Gumnjani, Mrakodol, Grdanovac and Tavija, and the municipality of Bosanska Dubica included: Strigova, Čitluk, Donja Slabinja and Babinac. Thus, Bosanska Kostajnica became only a local community for 32 years.
At the session of the National Assembly of the Republika Srpska on December 17, 1992. Bosanska Kostajnica changed its name to Srpska Kostajnica. The Municipality of Kostajnica was constituted by the decision of the National Assembly of the Republika Srpska in 1994, and the first session of the Assembly of the Municipality of Kostajnica was held on April 2, 1995. Compared to the period of 1962, the municipality was deprived of five villages: Strigova, Čitluk, Donja Slabinja, Bačvani and Babinac, which now are part of the municipality of Kozarska Dubica. In 2004, the Constitutional Court of Bosnia and Herzegovina ruled the name Srpska Kostajnica unconstitutional and returned the name Bosanska Kostajnica. In 2009, with the new Law on Territorial Organization of the Republic of Srpska, Bosanska Kostajnica became just Kostajnica.
