Mertens' agent Søren Lerby confirmed on 16 June 2013 that the player had agreed to sign with Napoli. He became Rafael Benítez's first signing as new Napoli manager after a reported fee of €9.5 million was agreed with PSV. He made his Serie A debut on 25 August 2013, coming on as a second-half substitute for Marek Hamšík in a 3–0 win over Bologna. His first goal for Napoli came on 30 October when he netted the winner in a 2–1 away victory over Fiorentina. In Napoli's first game following the Serie A winter break on 6 January 2014, Mertens scored his first brace for the club in a 2–0 win over Sampdoria. He then scored Napoli's second goal in their 2–0 win over Juventus on 30 March, helping inflict only the second league defeat over the eventual champions. In the final of the Coppa Italia on 3 May 2014, Mertens replaced Marek Hamšík in the 64th minute and went on to score in stoppage time, securing a 3–1 win over Fiorentina.