The 23-year-old striker has made a permanent move to Ewood Park, where he scored 12 goals during a season-long loan three years ago, on a four-year deal.
Gallagher, who came through the Southampton academy and made his first-team debut for the Saints in 2013, made 26 senior appearances during his time with the south coast club.
The forward, who scored the winner in the Under-21 Premier League Cup final in 2015, also spent time on loan at MK Dons and Birmingham City as well as the Ewood Park side.
Here is what some of the Saints faithful made of Gallagher's move away from the south coast and to the former Premier League champions on the social media platform:
Wishing you all the very best for the future Sam, go and score some goals— Art68⚪️ (@ArthurCogger) 13 July 2019
A pro, a gent, a decent player. See you back at St Mary's soon mate.— Ralph's Revolution (@saintbannerman) 13 July 2019
Should have been given a decent chance after Pelle left but didn’t happen for some reason. Timing is right and a good move for all parties. All the best.— Glen de la Cour (leagueoneminus10) (@L1minus10) 13 July 2019
Very best wishes @SamGallagher40 in your future career. I remember well that screamer you scored against @Rovers. Ewood is one of the grounds I haven’t been to so I’ll come and see you score one or two later in the season— Andrew (@angloscot4) 13 July 2019
All the best Sam! You've joined a great club— Southampton Page (@SouthamptonPage) 13 July 2019
All the best @SamGallagher40, great potential, decent club to sign for. All the best Sam and Rovers— ⚜️ The Saint ⚜️ (@thesotonsaint) 13 July 2019
All the best big guy time to boss it at rovers and show people what you can do— Joe (@sfcJoe_) 13 July 2019
Gallagher, who according to Lancs Live also had reported interest from Wigan Athletic and Brentford, becomes Blackburn manager Tony Mowbray's third summer signing, following on from the captures of midfielders Stewart Downing and Bradley Johnson.
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