Previously unranked, the 29-year-old from Sudbury, Ont., was listed at No. 15 in the ratings released Monday.
Gagnon (12-2) won his fourth straight in the 135-pound division when he submitted Salazar in the first round of the UFC's Fight Night card Saturday in Halifax.
Rory (Ares) MacDonald, a native of Kelowna, B.C., who fights out of Montreal, remained No. 2 in the welterweight ranks after his Halifax TKO win over Tarec Saffiedine, who was also unchanged at No. 9.
UFC middleweight champion Chris Weidman moved to No. 4 in the pound-for-pound rankings, passing heavyweight title-holder Cain Velasquez. Welterweight champion Johny Hendricks jumped past women's bantamweight title-holder (Rowdy) Ronda Rousey into the No. 8 spot, while bantamweight champion T.J. Dillashaw leapfrogged 135-pound rival Renan Barao to take over at No. 10.
The top three were unchanged with light-heavyweight champion Jon (Bones) Jones ahead of featherweight title-holder Jose Aldo and flyweight champion Demetrious (Mighty Mouse) Johnson.
The weekly UFC rankings are voted on by a media panel.