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. Mertens claimed another winners’ medal on 22 December 2014 after Napoli’s penalty shoot-out victory over Juventus in the Supercoppa. Despite being predominantly used as a substitute, he continued his fine form for Napoli over the course of the next season and a half, netting 10 goals and contributing 11 assists across all competitions in 2014–15, and 11 more goals in 2015–16. His tally in the latter campaign included a brace in Napoli’s joint-largest ever winning margin in the Europa League, a 5–0 win over Belgian side Club Brugge. Mertens also scored a second-half hat-trick in a convincing 6–0 victory over Bologna on 19 April 2016. Following the sale of Gonzalo Higuaín – the previous season’s Capocannoniere – to rivals Juventus, and a serious injury to his replacement Arkadiusz Milik, Mertens assumed a more senior role in the centre of Napoli’s attack for the 2016–17 Serie A season. On 11 December 2016, he netted a hat-trick in a 5–0 Serie A victory over Cagliari before taking it a step further in the next match by scoring four goals in a 5–3 win over Torino. Mertens’ four goals, which included the fourth fastest hat-trick in Serie A history, was the first haul scored by a Napoli player since Beppe Savoldi in 1977. It also saw him become the first player to score successive league hat-tricks since Juventus’ Pietro Anastasi in 1974, and the first to score seven goals across two matches since Antonio Angelillo in 1958. His form for club and country was rewarded when, on 30 December, he was named Belgian Footballer of the Year for 2016, beating Chelsea’s Eden Hazard and Radja Nainggolan of Roma to the award. On 4 February 2017, Mertens and Hamšík both scored hat-tricks in a 7–1 win over Bologna, a result which saw Napoli score seven away goals in a match for the first time in the club’s history. On the final day of the season he scored his 50th league goal for Napoli, and 28th for the campaign, ultimately ending one goal behind Golden Boot winner Edin Džeko. He maintained his goal scoring form the following season during which he scored 22 goals for the campaign.