Johnson, a 19-year-old from Melbourne's Xavier College, is a key position player, selected at pick 57 in last year's national draft.
He joins a growing list of former Xavier students in the Swans' team, that includes last year's NAB Rising Star winner Dan Hannebery (the other Xavier boys are Josh Kennedy and Ted Richards).
Johnson and Hannebery were in the same year at Xavier and were teammates in under 18 TAC Cup team Oakleigh Chargers.
Coach John Longmire told sydneyswans.com.au that Johnson was "super athletic and very competitive".
"Alex has been super, his attitude is unbelievable for someone of his age," Longmire said.
"He’s young so he’s going to make mistakes but we’re looking forward to seeing what he can achieve.’"
Johnson said: "When I got to Sydney last year I was determined to be as professional as possible and take advantage of all the support that was on offer.
"I was really looking forward to being a full-time footballer. I really tried to attack the pre-season and I had an aim to play a game this season but it’s come about already and I’m so excited by it.
"I can’t wait to get out there on Saturday."
Alex Johnson is $92,500 defender/forward in the Toyota AFL Dream Team competition
