Smith has signed a three-year deal with the Tigers, where he will work alongside another former Hawks assistant, senior coach Damien Hardwick.
The 46-year-old will take charge of defensive structures and strategies at Punt Rd.
Smith spent seven seasons at Hawthorn, starting as an opposition analyst in 2005 before moving into the defence coach role the following year.
"Ross is an excellent communicator, a great teacher and he's been involved in footy a hell of a long time," Hardwick said.
"Importantly, he knows about the system that we're trying to implement.
"The added bonus is that he's been involved in a premiership as an assistant coach at Hawthorn.
"He's had the experience, similar to me, of how to build up a side and get them to where we need to go."
A statement on hawthornfc.com.au on Wednesday said that Smith would leave with the club's blessing.
"After seven years of service, Smith departs Hawthorn on good terms having expressed a desire for change and new opportunity, which the club supported," the statement said.
Hawks coach Alastair Clarkson thanked the departing assistant for his contribution.
"We will miss his insight, his sense of humour and his outstanding work with our players and coaches," Clarkson said.
"I would like to thank him for his excellent contribution to Hawthorn and wish him and his family well in the future."
Smith played 224 games for North Melbourne between 1984 and 1996.
He had coaching stints at the Western Jets, the Kangaroos, Geelong and the Bendigo Diggers before joining the Hawks under former teammate Clarkson.
