This article reconstructs Jacob van Heemskerck's second voyage to the East Indies and his capture of the Portuguese merchantman Santa Catarina on 25 February 1603. It incorporates important new archival evidence like Van Heemskerck's letter to the directors of the Dutch East India Company of 27 August 1603, and the original text of the verdict of the Amsterdam Admiralty Court, which confiscated the Santa Catarina on 4 September 1604. It has long been known that the Dutch jurist Hugo Grotius (1583-1645) wrote De Jure Praedae in defense of the ship's seizure and at the explicit request of the directors of the Dutch East India Company. Historians have failed to recognise, however, that Grotius' conceptualisation of natural rights and natural law in De Jure Praedae is based to a large extent on Van Heemskerck's own justification of privateering. A key notion of Grotius' rights theories - the individual's right to punish transgressors of the natural law in the absence of an independent and effective judge - follows logically from Van Heemskerck's reasoned decision to assault the Santa Catarina in revenge for Portuguese mistreatment of Dutch merchants in the East Indies. As shown by recent work in international relations theory - notably Edward Keene's Beyond the Anarchical Society: Grotius, Colonialism and Order in World Politics (Keene, 2002) - the natural law and natural rights theories that Grotius formulated in De Jure Praedae cannot be divorced from Dutch imperialism and colonialism in the early modern period.