The Moldavian Magnate Wars, or Moldavian Ventures, refer to the period at the end of the 16th century and the beginning of the 17th century when the magnates of the Polish–Lithuanian Commonwealth intervened in the affairs of Moldavia, clashing with the Habsburgs and the Ottoman Empire for domination and influence over the principality.

Jan Zamoyski, Polish grand crown chancellor (kanclerz) and military commander (grand crown hetman), known for his opposition towards the Habsburgs, had been a vocal supporter of Commonwealth expansion in the southern direction. Since the early plans made by Commonwealth King Stefan Batory for the war against the Ottomans, Zamoyski supported them, viewing those plans as a good long-term strategy for the Commonwealth. Any policy that was against the Ottomans was also supported by the Holy See, and Pope Sixtus V strongly expressed his support for any war between the Commonwealth and the Ottomans. Three powerful magnate families from the Commonwealth, the Potockis, Koreckis and Wiśniowieckis, were related to the Moldavian Hospodar (Prince or Voivode) Ieremia Movilă (Jeremi Mohyła), and, after his death in 1606, they supported his descendants.

Around the end of the 16th century, relations between the Commonwealth and the Ottomans, never too cordial, further worsened with the growing number of independent actions by Cossacks. From the second part of the 16th century, Cossacks started raiding the territories under Ottoman rule. The Commonwealth could not control the fiercely independent Cossacks, but was held responsible for them, since at that time they were nominally under the Commonwealth rule. At the same time, Tatars living under Ottoman rule were raiding the Commonwealth. However, they attacked mostly in the south-eastern areas of the Commonwealth, which were fairly sparsely inhabited, while the Cossacks were raiding the heart of the Ottoman Empire, wealthy merchant port cities just two days away from the mouth of the Dnieper river (which the Cossacks used as their main transportation route). By 1615, Cossacks had even burned the townships on the outskirts of Constantinople. Consecutive treaties between the Ottoman Empire and the Commonwealth called both parties to curb Cossack and Tatar activities, but they were never implemented on either side of the border. In internal agreements, pushed forward by the Polish side, the Cossacks agreed to burn their boats and stop their raiding. However, Cossack boats could be built quickly, and the Cossack lifestyle required periodic hunts for glory and booty. Sometimes Cossacks just needed resources to ensure their subsistence, while on other occasions they were bribed by the Habsburgs to help ease Ottoman pressure on their borders. Also, there was widespread animosity between Cossacks and Tatars, after decades of border clashes and reciprocal looting of estates and villages. Cossacks raided Ottomans territories and their vassals near the Black Sea almost yearly, usually attracting retaliatory Tatar raids (and vice versa). The vicious circle of chaos and retaliations often turned the entire south-eastern Commonwealth border into a low-level warzone.

In 1593, war between the Ottoman Empire and the Habsburgs started. In 1594 a very strong Tatar raid, carried out by about 20,000–30,000 men led by the Khan of Crimea, Ğazı II Giray (Gazi Gerej II), plundered Pokucie and moved to Hungary through mountain passes, in order to plunder Habsburg lands. Commonwealth troops gathered too late to intercept it. The Prince of Transylvania, Sigismund Báthory (Zsigmond Báthory), nephew of former Polish king Stefan Batory (István Báthory), had strengthened Habsburg influence in Moldavia after setting Ștefan Răzvan (Stefan Rozwan) on the Moldavian throne. Ștefan Răzvan was a Roma from Wallachia (his father had been an Ottoman Muslim Roma, therefore not a slave) and had married a Moldavian noblewoman (his story was the basis of a play by 19th-century Romanian writer and historian Bogdan Petriceicu Hasdeu).

A pro-Polish hospodar was mostly tolerated by the Porte when the Commonwealth was anti-Habsburg or neutral. Therefore, when Emperor Rudolf II's forces gained control of Moldavia, Transylvania (Polish: Siedmiogród) and started supporting Mihai Viteazul (Polish: Michał Waleczny), prince of Wallachia, the Ottomans didn't look too favourably at the Commonwealth's meddling.

In 1595 Zamoyski, persuaded by Moldavian refugees, decided to intervene. The Commonwealth forces (numbering ~7,000–8,000 soldiers) under hetman Jan Zamoyski crossed the Dniester, defeated local opposition (while Transylvanian troops retreated to their own country) and Ottoman reinforcements, and set Ieremia Movilă on the Moldavian throne as a Commonwealth vassal. This was seen by many as a very dangerous step because the Ottomans were preparing to place their own candidate on the Moldavian throne. Zamoyski contacted grand vizier Sinan Pasha and negotiated with the Ottoman governor on the Black Sea island of Tyahyn (near the Dnieper river) and convinced them of his peaceful intentions and that he did not want to fight with the Ottoman Empire. However, the Khan of Crimea, Ğazı II Giray, reacted and entered Moldavia with about 20,000 men (but no cannons and few janissaries). Zamoyski fortified his camp near Cecora at Prut river, withstood a three-day siege (17–20 October), and managed to obtain an agreement with the Ottoman Empire that recognized Movilă as hospodar (Treaty of Cecora). Moldavia became the Commonwealth's vassal and paid tribute to Constantinople at the same time (this is known as condominium—territory under rule of two sovereign powers). Not satisfied with this, previous hospodar Ștefan Răzvan invaded Moldavia, but his troops were crushed by Zamoyski and Răzvan was impaled by Movilă.

In 1599, Mihai Viteazul, wishing to secure his back after Sigismund Báthory's departure from the Transylvanian throne, defeated the new ruler of Transylvania, Andrew Cardinal Báthory (Andrzej Batory), who lost his life fleeing after battle, and took over Transylvania as governor on behalf of the Habsburg emperor Rudolf II. Later on, Mihai defeated Ieremia Movilă and took control over almost all of Moldavia, with the exception of Khotyn (Chocim or Hotin, a castle and a city on the right bank of the Dniester), which remained in Polish hands. Mihai used titles of voivode of Wallachia, Transylvania and Moldavia for the first time in May 1600. He tried to get recognition from Emperor Rudolf II, offered his vassalage to the Commonwealth, and organized an anti-Turkish league. After King Sigismund III Vasa (Zygmunt III Waza) refused, Mihai sent his troops to take over Pokucie (an area Moldavians were claiming to be theirs) but Commonwealth hetman Stanisław Żółkiewski met them with resistance.

In 1600 Zamoyski and hetman Jan Karol Chodkiewicz gathered Commonwealth forces, returning to Moldavia, where they fought Mihai. Zamoyski defeated Mihai Viteazul near Bucov (Bukova) in Wallachia, on the Teleajen river, near present-day Ploiești, restored Ieremia Movilă to the throne, and helped his brother Simion Movilă to gain the throne in Bucharest, thus temporarily extending the sphere of Commonwealth influence south all the way to the Danube. In the meantime, Mihai Viteazul traveled to Vienna to ask for the Emperor's help, in exchange for assisting the Habsburgs against the Ottomans and Imperial influence over Moldavia, previously aligned with the Commonwealth. The Emperor promised help and in 1601 sent an army led by Giorgio Basta that was to accompany Mihai on the way back. Upon their arrival in Transylvania, after the joint victory at Gurăslău against the prince of Transylvania, Sigismund Báthory, general Basta assassinated Mihai Viteazul during the night, on the field of Câmpia Turzii (south of Cluj), effectively taking Transylvania under the Emperor's lead.