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.
У периодима турске владавине становништво овог краја је подизало бројне буне. Најпознатија је Пецијина буна 1858. године коју су подигли хајдуци Петар Поповић Пеција и Петар Гарача. Петар Поповић, звани Пеција (1826—1875) је рођен од оца Петра и мајке Илинке у селу Бушевић у Крупској. Већину живота провео је у граду Костајници. Петар је сам научио писати и читати, а осим српског говорио је и турски језик. Не признавајући отоманску државу Петар у 22. години одлази у хајдуке и од тада до краја живота активно се бори против турске државе. У јесен 1858. заједно са хајдучким харамбашом Петром Гарачом из Стригове, диже велику буну у Кнешпољу и пред Босанском Костајницом гради шанац познат као „Пецијин шанац“. Хајдуцима се придружује и народ, па се на шанцу одиграва жестока битка између Турака и устаника. Након дуге и тешке борбе Турци су успјели устанике натјерати преко ријеке Уне у Хрватску Костајницу гдје се већина предала аустријским стражама. Пеција и Гарача са још око 300 устаника нису се предали већ су се повукли у Пастирево. Ова буна је у народу позната као Костајничка или Пецијина буна, а рјеђе као Кнешпољска буна. Послије Костајничке буне Турци су два пута хватали Пецију. У Цариграду је био осуђен на смрт, са пресудом да буде погубљен у свом крају јер је „тамо највише зла починио“. На путу у родни крај Пеција је побјегао Турцима код Крагујевца, гдје се и задржао и добио мјесто стражара у тополивници. Ту је остао све до новог устанка у Босанској Крајини 1875. године. Приступио је устаницима и већ почетком септембра учествовао у бици код села Гашице, садашње Гашнице. Тога дана Петар Пеција се још једном показао као способан вођа устаника надјачавши Турке и нагнавши их у бијег. Међутим, недуго послије, а по народном предању због издаје неког Илије Чолановића, Турци су изненадили устанике и након дугог и жестоког боја разбили су хајдуке од којих су неки побјегли у Просару, а њих педесетак са Пецијом на челу су остали да се боре на обали Саве. Потјерани турском силом Пеција и устаници успјели су се докопати лађе и прећи преко ријеке, али без заклона, изложени турској пушчаној ватри готово сви изгинуше. Само Пеција и два човјека остадоше живи, али Петар, немирног и храброг духа, умјесто да се склони, изађе на чистину и повика на Турке: „Ево Турци, нека знадете, Пеције нисте убили, још је жив, и наскоро ће Вам се осветити“. Тог момента га је погодило турско тане и на мјесту га убило. Било је то 10. септембра 1875. године. Међу народом је кружила прича да га није убило пушчано зрно већ да је пушка била набијена пшеницом. Мртвог Пецију са Корманошем и осталим јунацима другови сахранише у селу Јабланцу ниже Јасеновца, а десет година након погибије Пецијине су кости пренешене и похрањене код манастира Моштаница подно Козаре, близу Козарске Дубице.
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.
