Galloway has almost 80 games of A-League experience since his debut in 2013 and won a Premiers' Plate and championship while at Melbourne Victory.
Phoenix coach Darije Kalezic and assistant coach Rado Vidosic moved swiftly to sign the 22-year-old who impressed during his time at Victory and last season for Central Coast.
"His versatility and ability to play both left and right fullback makes him a very important player for our team," Vidosic told wellingtonphoenix.com.
After trials in the Netherlands, Galloway arrived in Wellington on Monday and after passing his routine medical will begin training with the Phoenix later in the week.
"It was a great experience trialling abroad but [Rado and Darije] convinced me that this would be the best place for me, to develop my game," Galloway told the club's official website.
"The fact that Darije has coached in Netherlands, and brought through some top young players means I'm confident if I work hard, he can help take me to where I need to be."
Galloway is the second signing since the new coaching team conducted their review of the Wellington squad and follows goalkeeper Lewis Italiano signing a one-year deal to return to the club.
