The article presents conversions of Jews to Catholicism in Gdansk and its surroundings. Information on the converts comes from Jesuit reports ('litterae annuae') and from those written for the 'Congregatio de Propaganda Fide' of the Holy See (the so-called 'Fructus Missionis'), which were sent to Rome by the Gdansk College of the Jesuits. The conversion issue was highlighted in the article against a background of achievements of other Jesuit centres in Royal Prussia and in relation to the activity of the Society of Jesus in the whole of the Commonwealth of Poland and Lithuania. For comparison, the context of Evangelical missionary expeditions in Poland, run by the 'Institutum Judaicum et Muhammadicum' in Halle, was also taken into consideration. The first Jewish convert was mentioned in the reports of the Gdansk College of the Jesuits in 1674. However, at the turn of the 17th and 18th century conversions from Judaism to Catholicism were extremely rare. A higher number of that kind of conversions took place in 1719-1723, 1728-1733 and 1752-1756, when between one and two cases per year were noted. The greatest number of converts were newcomers from outside Gdansk and Royal Prussia, people going through difficult periods of their lives, those in conflict with their communities and minors deprived of their guardians for various reasons.