Carl Robinson's Wanderers could lose players -- including striker Mitch Duke -- to Socceroos or Olyroos duties later in the season and McDonald, who had a long career overseas with the likes of Celtic, Middlesbrough and Motherwell, adds goal-scoring experience.
"I tried to come to this great club about four years ago while I was at Motherwell, but unfortunately that move [didn't happen] so I am happy that I've finally been able to make the move," McDonald said.
"We got here eventually, you don't expect to have three clubs in two years but this is a club I've wanted to come to for a long time.
"Coming to the Wanderers and being with a proven manager, I know what I am coming to, I know the expectations and I'm really excited for the opportunity ahead."
The eighth-placed Wanderers are in the midst of a six-game winless streak, losing their past two, to be two points adrift of the the top six.
"There are leadership qualities I can bring, to the players and to the group, to help the manager on the pitch and deliver what the manager wants," McDonald said.
"I want to lift the place, it always happens when you bring someone in, I think it is healthy and hopefully I can do the business."
Robinson expected the 37-year-old to deliver some much-needed on-field drive.
"I am delighted to welcome Scott to the Western Sydney Wanderers and he brings some fantastic leadership qualities to the club," Robinson said.
"Every game is huge for us now at the business end of the season and we need to do everything we can to change our current momentum."