Smith, who most recently signed to second-tier Scottish Championship side Dumbarton, but was loaned to third-tier League One side Cowdenbeath, made 24 appearances for Hearts in the Premier League from 2009 to 2013.
Smith will line up for the Jacks when the pre-season night series gets underway next month.
The 24-year-old said it was “now or never” to move to Australia and was looking forward to playing football in weather more favourable than the torrential conditions Scotland often had.
“A couple of months ago when we played Arbroath the weather ruined the game,” he said.
“The ball was off the pitch every 30 seconds and when the weather’s like that it’s hard to play football.”
ECU secured the signing through a club connection to Smith’s father, who played with Jacks’ youth coach Willie Herd at Falkirk Football Club.
Smith will also have a coaching role at the club.
“I’ve loved playing in Scotland but I’ve always wanted to try playing somewhere else, in a different country,” he said.
The signing buoyed ECU coach Dale McCulloch with the Jacks having not had much success in recent seasons.
“Everyone knows we struggled in front of goal for the last two seasons, and Gordon will give us a massive boost in the final third,” he said.
“He comes with a wealth of experience in Scottish football and his eye for goal will be vital.”
The Jacks have also signed former Perth Glory footballer Steven McGarry in a further boost to their stocks.