Hardwick's long-expected deal was confirmed by the Suns in a statement on Monday morning.
Hardwick quit as coach of Richmond in May, citing burnout after leading the club to three premierships.
"I firmly believe this is the beginning of an incredible journey with a club and a community that has a burning desire to succeed," Hardwick said in a statement.
"The Gold Coast deserves a successful team it can be proud of and it's now my job to deliver that.
"I can see the tremendous potential we have in front of us but understand it will take hard work from here to reach our goals.
"The foundations are in place for us to build this club into a contender for years to come and this is a challenge I am extremely excited to take on."
The Suns, who joined the AFL in 2011, have never reached the finals and sacked Stuart Dew as coach on July 11.
Gold Coast chairman Bob East said the club was delighted to secure Hardwick, who coached the Tigers from 2009.
"We have been unequivocal in our desire to deliver success for our football club, for our loyal and growing group of members and fans, and to the wider Gold Coast community," East said in a statement.
"This appointment is clearly a significant step in that direction.
"When we began the process to determine our senior coach, it was quickly apparent Damien was the best candidate available.
"We have high expectations for our club and firmly believe Damien can deliver on these ... to achieve premiership success."Â
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