Yet to prove himself in Australian football, the young stopper will have another chance demonstrate his worth after returning from a spell overseas
Gallagher, 23, has agreed a one-year deal and joins from Hanoi T&T, having made 11 appearances - scoring twice - at the Vietnamese club last term.
The defender came through the youth ranks at Sydney FC and made his A-League debut for Central Coast Mariners, before joining Melbourne Victory.
"I always had it in my mind that if had the chance to play in Australia again I'd look to try and take it, so when the opportunity to trial with Newcastle came up I jumped at the chance," Gallagher said.
"Thankfully the trial went well and I was able to get a contract. I'm excited to be a Jets player and really looking forward to getting a good pre-season under my belt and giving everything I can for the club.
"I'm lucky enough to know a few of the boys from previous experiences and the rest of the squad has been really good to me since I came in on trial, so it's made the transition nice and easy."
New Jets coach Phil Stubbins believes the former Australia Under-20 international can prove a versatile option at the club next season.
"Sam brings a real balance to the squad as a player who can fill the role of left stopper or left fullback," Stubbins said.
"I think he's got a lot of confidence from the experience he's had in Asia and he's learnt a hell of a lot from his time over there as well.
"He's a good kid who is hungry for success and we're pleased to have him in Newcastle."
Gallagher follows Jonny Steele, Marcos Flores, Adrian Madaschi, Josh Barresi and Edson Montano in joining the Jets for the 2014-15 campaign.
Joey Gibbs, Josh Mitchell, James Brown, Connor Chapman and last season's captain Ruben Zadkovich have all left the Hunter Stadium outfit.