The Belgium international clinically finished a swift counter-attacking move to bring his side level at 1-1 in the second leg of their Coppa Italia semi-final against Inter.
His equalizer in the 41st minute, set up by Lorenzo Insigne's square pass, canceled out Christian Eriksen's early opener at the San Paolo.
Mertens matched Hamsik's career tally of 121 for the Italian club with the only goal in the 1-1 Champions League draw with Barcelona on February 25.
The 32-year-old, who joined from PSV in 2013, is out of contract at the end of the season but is expected to sign a new deal to continue his career with Napoli.
Mertens passed Diego Maradona (115) to move into the second spot on the list last year. His effort against Inter takes his tally for the season in all competitions to 13.