Jay Rodriguez notched a double either side of David Cano's equaliser to give Mauricio Pochettino's side a narrow but comfortable victory in Palafrugell.
The Saints gaffer had 21 players at his disposal, with Emmanuel Mayuka and Jos Hooiveld missing out with knocks while Maya Yoshida was not yet ready to feature having only begun pre-season training on Monday.
With Saints in their new home strip, the fixture – hosted at the Estadi Josep Pla i Arbonés, home of Llagostera's neighbours FC Palafrugell – got underway at 7pm local time.
After a speculative effort from Llagostera's Pitu in side the first minute, Saints quickly settled into their rhythm and moved into the ascendancy.
Rodriguez looked the early danger-man, and he saw a 12th-minute goal chalked off when he converted Calum Chambers' low centre only to be ruled offside.
Jason Puncheon then had a close-range effort saved by Llagostera goalkeeper Moragón, before José Fonte thumped a header onto the crossbar from James Ward-Prowse's out-swinging corner.
The breakthrough arrived on 28 minutes, and it came in rather calamitous circumstances. Llagostera defender Pere Sagarra overhit his pass back to Morágon and, after the ball had run under the stopper's foot, RODRIGUEZ outpaced him and slammed in the opener.
The striker went in search of a second shortly afterwards but saw his attempted lob drop wide of the far post, before turning provider for Steven Davis only for the midfielder to miscue his volley.
The pair combined again six minutes before the break as Davis slipped the ball to Rodriguez who, after rounding Moragón, ran out of space and the ball rolled out for a goal kick.
HALF TIME: UE LLAGOSTERA 0-1 SOUTHAMPTON
Both sides made ten changes each at the interval, with only goalscorer Rodriguez remaining on the field for Saints.
The first chance of the half went Saints' way four minutes after the break as Adam Lallana picked out Rickie Lambert, but his header was saved by substitute goalkeeper Nico Ratti.
Skipper Lallana then stung the palms of the Llagostera stopper barely a minute later with a fierce drive that was gathered at the second attempt.
However, having failed to threaten at all in the opening hour, the hosts got themselves on the scoresheet on 61 minutes as defender CANO headed Adriano García's corner past Paulo Gazzaniga.
But parity was to be short-lived as, after Lallana had seen a shot parried by Ratti, RODRIGUEZ was on hand to slide in from a tight angle to make it two-one.
That was to be the two-goal striker's last involvement, as he was immediately replaced by the returning Ward-Prowse with 68 on the clock.
New boy Victor Wanyama tried his luck with a curling effort on 78 minutes, before Llagostera made a triple change in an attempt to force a late leveller.
Saints remained in the ascendancy as the full-time whistle drew closer, with a corner from Luke Shaw being headed over by Wanyama before Llagostera substitute Fullana fired off target at the other end.
A good interchange between Ward-Prowse and Lallana was then foiled by Ratti, before Wanyama sent a thumping half-volley wide of the hosts' goal.
In the end, despite the hosts briefly drawing level in the second half, Saints were good value for their win on a warm evening in Catalonia.
FULL TIME: UE LLAGOSTERA 1-2 SOUTHAMPTON
UE Llagostera team: Moragón (Nico Ratti 46), Barón (Barnils 46), Vallho (Toni Cunill 46), Pere Sagarra (David Cano 46), Enric Pi (Marc Garcia 46), Tito (Carles Estruch 46 (José Flores 78), Isaac Maldonado (Anas El Morabet 46 (Fullana 78), Jordi Masó (Pere Sagarra 68), Eloy Gila (Adriano García 46 (Pol Mitjà 78)), Pitu (Aumatell 46), Ritxi Mercader (Nuha 46). Substitutes: Christian Tomás, Adrià Muñoz.
Goal: David Cano (61)
Southampton team: Kelvin Davis (Paulo Gazzaniga 46), Calum Chambers (Nathaniel Clyne 46), José Fonte (Jack Stephens 46), Jordan Turnbull (Dejan Lovren 46), Danny Fox (Luke Shaw 46), Jack Cork (Victor Wanyama 46), Steven Davis (Morgan Schneiderlin 46), James Ward-Prowse (Lloyd Isgrove 46), Jason Puncheon (Adam Lallana 46), Jay Rodriguez (James Ward-Prowse 68), Guly (Rickie Lambert 46).
Goals: Jay Rodriguez (28 & 67)