The siege of Gvozdansko (Croatian: Opsada Gvozdanskog) was an Ottoman siege of the fort of Gvozdansko in the Kingdom of Croatia in 1577–1578. In the 1570s, the Ottomans intensified their efforts to capture the valley of the Una River. A string of forts along the Una, centred around Gvozdansko and in possession of the Zrinski noble family, formed the main line of defense of Croatia since 1527. The fort held off Ottoman attacks in 1540 and 1561.

In 1575, Ferhad Bey Sokolović, Sanjak-bey of Bosnia, began an offensive on central Croatia. By the end of 1576, the defense system on the Una collapsed and most of the forts in the valley were captured, including Bužim and Cazin. While Gvozdansko withheld an attack in June after three days of fighting, the attack left the walls of the fort damaged. Ferhad Bey renewed the campaign in September 1577 and attacked the remaining Croatian-held forts in the area. The Ottoman army besieged Gvozdansko on 3 October, whose garrison numbered 300 soldiers and miners, under the command of Damjan Doktorović and three other Croatian captains. With the capture of the nearby fort of Zrin on 20 December, Gvozdansko was left completely isolated. Due to a lack of troops, there were no attempts to relieve Gvozdansko.

The final phase of the siege began in early January 1578. Around 5,000 Ottoman troops were involved in the siege, and a similar number were positioned at the approaches to Gvozdansko. Calls for the surrender of the fort were rejected. Three major Ottoman assaults were repelled from 10 to 12 January. By the final day of the siege, on 13 January, the entire Croatian garrison was dead and Ottoman forces entered the fort in the early morning. Gvozdansko and most of the Una Valley were briefly recaptured by Croatian and other Habsburg forces during a counter-offensive in the summer of 1578. Their gains were annulled by Ferhad Bey in late September. The Ottoman Empire controlled Gvozdansko until 1685. Ottomans recaptured it in 1690. But, it was definitely captured by Austrians in 1718.

In 1526, Ottoman forces led by Sultan Suleiman the Magnificent defeated a large Hungarian-led Christian force at the Battle of Mohács. As King Louis II died in battle and had no heir, both the Kingdom of Hungary and the Kingdom of Croatia, which were in a personal union, became disputed territories between Ferdinand I, Archduke of Austria, from the House of Habsburg, and John Zápolya, Voivode of Transylvania. A civil war ensued, which enabled the Ottomans to gain more territory in Hungary. Most of the Croatian nobility backed the Habsburgs, expecting aid in the wars with the Ottoman Empire. Zápolya received military assistance from Suleiman.

Following the fall of the Banate of Jajce in 1527, the main line of Croatian defense moved to the valley of the Una River, where Gvozdansko was one of the main fortifications. Gvozdansko was a rectangular fort in possession of the Zrinski family, located on the main road between the towns of Dvor and Glina. It had four towers on its corners and a larger tower opposite the entrance, which had four floors and was 10 meters in diameter. The fort had two longer walls around 25 meters in length, and two shorter walls with a length of around 15 and 18 meters. Gvozdansko was first mentioned in 1488 and was built to protect the nearby mines of iron, copper, lead, and silver. The extraction of silver in Gvozdansko began in the late 15th and early 16th centuries, when Nikola III, who usually resided in Gvozdansko, was the head of the Zrinski family. The Zrinskis also minted their own silver coins in Gvozdansko. The silver mines in their possessions yielded an annual income of 30,000 florins, according to reports from Nikola III. With the growth of their wealth and influence, the family gained more land in the valley of the Una, such as the towns of Kostajnica, Krupa, and Dubica, on the right bank of the Una. The fort of Zrin, which gave the name to the Zrinski family, was located northeast of Gvozdansko. In 1534, Nikola III died, and his wife Helen, and two sons, Nikola IV (commonly known as Nikola Šubić Zrinski) and Ivan I, took the role of governing the family possessions.

There were no major incursions into Croatia from 1534 to 1536. In the summer of 1536, Ghazi Husrev Bey, Sanjak-bey of Bosnia, began an offensive north of the Sava River. By January 1537, he captured the city of Požega. In the space of six months, over half of Slavonia was under Ottoman control, from Valpovo to the fort of Jasenovac at the mouth of the Una into the Sava. Husrev Bey then attacked the lower Una and captured Dubica in January or early February 1537. Shortly after the fall of Dubica, the Croatian nobility met in Zagreb to evaluate their defensive capabilities, and pointed out the lack of military forces and financial difficulties. The Zrinskis started enrolling their miners as soldiers, and were able to take back Dubica in the same year. The Ottomans regained initiative in the war following the failure of Johann Katzianer's campaign in Ottoman-held Slavonia in October 1537. They recaptured Dubica in May 1538. As Suleiman was busy with the war with Venice, the activities on the Ottoman–Croatian border calmed down until October 1539, when Zrinski had Katzianer executed for siding with Zápolya and negotiating the surrender of Kostajnica to the Ottomans. Suleiman threatened retaliation if Zrinski was not punished by Ferdinand.

The Habsburgs managed to negotiate a six-month peace treaty in Constantinople, starting with 1 January 1540, but clashes on the border did not stop. Croatian forces led by Zrinski again took control of Dubica in late January 1540. The Ottomans responded with an attack on Pakrac. Croatian Ban Petar Keglević sent an army to help Zrinski, but immediately called it off at the request of Queen Anne, in order not to violate the agreed armistice. A retaliatory attack against Zrinski was carried out by Husrev Bey, Murat Bey Tardić, Sanjak-bey of Klis, and Mehmed Bey Jahjapašić from the Sanjak of Smederevo. The main targets were Kostajnica, which was briefly captured, and Zrin and Gvozdansko, which held off attacks. The offensive, which began in early May, ended in mid June, on Husrev Bey's orders. The Zrinskis reported that all of the mines near Gvozdansko were destroyed. Following the death of Zápolya in June 1540, the war mainly took place in Hungary.

In late 1540, Ferdinand approved more money to Zrinski for military purposes. Two years later, Zrinski was named Ban of Croatia, in place of Keglević, who was removed from office for obtaining possessions of his daughter's late husband. Ottoman raids in the Una Valley continued in 1544. Zrinski transferred the family center from Zrin to Čakovec in the Međimurje region in 1546. In June 1547, a five-year peace treaty was concluded between Ferdinand and Suleiman. The treaty was respected on both sides. Rüstem Pasha, Grand Vizier of the Ottoman Empire, unsuccessfully tried to get Zrinski excluded from the treaty, due to his raids of Ottoman lands in earlier years.