A preliminary peace was concluded on 3 October 1735 with Austria. However, the peace was not finalised until three years later with the Treaty of Vienna (1738), ending the War of the Polish Succession.
Naples and Sicily were ceded by Austria to Charles, who gave up Parma and Tuscany in return. (Charles had inherited Tuscany in 1737 on the death of Gian Gastone.) Tuscany went to Emperor Charles VI's son-in-law Francis Stephen, as compensation for ceding the Duchy of Lorraine to the deposed Polish King Stanislaus I.
The treaty included the transfer to Naples of all the inherited goods of the House of Farnese. He took with him the collection of artwork, the archives and the ducal library, the cannons of the fort, and even the marble stairway of the ducal palace.
Charles' mother Elisabeth again began looking for potential brides for her son, now formally recognised as King of Naples and Sicily. It was impossible to get an Archduchess of Austria as a bride, so she looked to Poland, choosing Princess Maria Amalia of Saxony, a daughter of the newly elected Polish king Augustus III and his (ironically) Austrian wife Maria Josepha of Austria. Maria Josepha was a niece of Emperor Charles; the marriage was seen as the only alternative to an Austrian marriage.
Maria Amalia was only 13 when she was informed of her proposed marriage. The marriage date was confirmed on 31 October 1737. Maria Amalia was married by proxy at Dresden in May 1738, with her brother Frederick Christian of Saxony representing Charles. This marriage was looked upon favourably by the Holy See and effectively ended its diplomatic disagreement with Charles.
The couple met for the first time on 19 June 1738 at Portella, a village on the frontier of the kingdom near Fondi. At court, festivities lasted till 3 July. As part of the celebration, Charles created the Order of Saint Januarius — the most prestigious order of chivalry in the kingdom. He later had the Order of Charles III created in Spain on 19 September 1771.
Despite being an arranged marriage, the couple became very close. They had 13 children, some of them died in infancy.