Gonzalo Higuain 9 Argentina
Higuain's departure, which he said was not his decision, came shortly after the Turin side signed five-times world player of the year Cristiano Ronaldo from Real Madrid.
Juventus defender Mattia Caldara will also move to Milan as part of the complex three-way deal, the clubs said on Thursday.
Several dozen fans waited as Gonzalo Higuain arrived at a Milan hotel and Italian media said he would undergo a medical early on Thursday, with agreement having been reached between all parties.
Juventus' 1-0 derby win over Torino came at a high cost with both prized striker Gonzalo Higuain and his replacement, Federico Bernardeschi, coming off with injuries.
It was the Higuain's fifth goal in as many games against Napoli since he left the club in acrimonious circumstances last year.
Gonzalo Higuain has scored 101 goals in 153 Serie A matches for Napoli and Juventus.
A stunning volley from Stephan El Shaarawy gave AS Roma their third straight 1-0 win, this time at home to midtable Bologna in a father-versus-son encounter, in Saturday's other game.
Balotelli missed first penalty of his career after Britos,Higuain scored early in each half for visitors.
A stunning Insigne free kick proved for Napoli who narrowly beat 10-man Borussia Dortmund 2-1.
Goals from Gonzalo Higuain and Jose Callejon sent Napoli top of Serie A after they beat Atalanta 2-0