The A-League champion confirmed the 29-year-old, who spent the second half of the 2018-2019 season with Dundee, has joined on a two-year deal.
McGowan will play in the A-League for the first time in his career and could face his younger brother, Dylan, who has joined rivals Western Sydney Wanderers.
And the centre-back, who has spent much of his career in Scotland, is looking forward to teaming up with former Socceroos team-mate Alex Wilkinson.
"I'm excited to be coming back to Australia and to be playing my first Hyundai A-League football,” McGowan told Sydney FC's official website.
"It's a brilliant opportunity, especially to be joining the reigning A-League champions. Sydney FC's success in recent years speaks volumes about the type of club it is and I needed no convincing to sign.
"It will be great playing alongside Wilko again after our days with the Socceroos and at the World Cup in Brazil. I'm sure we will have no problem in quickly regaining our understanding."
On facing his younger sibling, he said: "It's going to be great to have my brother living in the same city once again. I've heard and seen so much about the Sydney derbies over the years and I can't wait to play in one.
"It will be extra special having Dylan play in the game as well. I'm also looking forward to getting another taste of AFC Champions League football again and performing well for the club in that."
McGowan's arrival comes after Steve Corica's side brought in former Wanderers attacking midfielder Alexander Baumjohann.
"At 29, Ryan is in the prime of his career and with so many high-level games under his belt, he is a huge asset to us," Corica added.
"He reads the game well, can play out from the back and is physically very quick and strong. He is a great signing who will fit into our vastly experienced defensive line-up with ease."