Graziano Pelle marked his Saints debut in fine style as he scored twice in a 6-0 romp over Belgian minnows Sporting Hasselt.
The 29-year-old striker, signed for a fee of around £7m, took just five minutes to notch his first goal as he converted Lloyd Isgrove’s cross.
He added his and Saints’ second on 32 minutes, this time turning home from Jason McCarthy’s low ball into the box.
It helped Saints on their way to another very comfortable win to round off their trip to Belgium, having already beaten Dutch amateur side EHC Hoensbroek 4-0.
Ronald Koeman once again changed his entire side at half time as he gave game time to as many players as possible, and protected them from the heat the match was played in.
Sam McQueen scored his second goal in as many games to make it 3-0 on 59 minutes before Hasselt defender Tom Weerts turned into his own net after Harrison Reed’s dangerous ball in.
Emmanuel Mayuka added another on 78 minutes as he coolly tucked away a penalty after Billy Sharp’s attempted run was cut off by a handball in the area.
The scoring was rounded off three minutes from time as Sharp’s blocked shot rebounded to Gallagher who slotted home.
Saints now return home to prepare for three away friendlies on English soil, starting with Swindon on Monday night, before they round off their preparations for the new season by taking on Bayer Leverkusen at St Mary’s.
Saints first half: K Davis, McCarthy, Stephens, Hooiveld, Targett, Cork, Ward-Prowse, S Davis, Tadić, Isgrove, Pellè.
Saints second half: Boruc, Chambers, Fonte, Yoshida, Turnbull, Flannigan, Reed, Sharp, McQueen, Mayuka, Gallagher.
Goals: Pelle 5, 32, McQueen 59, Weerts og 70, Mayuka pen 78, Gallagher 87