The 41-year-old allrounder last played professionally for the Sydney Sixers two summers ago and has since served as an assistant coach at the Thunder.
Christian has not played a professional match of cricket since the Sixers' BBL12 semi-final loss to the Brisbane Heat but has been keeping fit with UNSW in the NSW Premier Cricket competition.
He will join an elite club of players to feature in the BBL in their 40s that includes Brad Hodge, Peter Siddle, Fawad Ahmed and the late Shane Warne - the oldest player in league history at 43.
Christian has won BBL titles with the Sixers, Renegades and Heat. (Dean Lewins/AAP PHOTOS)
Christian is available for the remainder of the tournament, which has already featured big improvements from reigning wooden spooners the Thunder.
Christian's comeback follows injuries to Daniel Sams and Cameron Bancroft, both sidelined after a horror collision in the field during last week's win over the Perth Scorchers.
Sams (concussion) and Bancroft (fractured shoulderblade, broken nose, concussion) joined a mounting injury toll that has also sidelined Jason Sangha, Nic Maddinson and Tanveer Sangha.
In addition, the Thunder could lose Sam Konstas to Test duties later in the tournament and expect to farewell internationals Sherfane Rutherford and Lockie Ferguson prematurely as well.
"We're very fortunate to have Dan already within the club," said Thunder general manager Trent Copeland.
"He's a legend of the BBL and we've got no doubt he's still up to the level. We can't wait to see him jump back into this environment and make an impact."
Christian is considered a T20 specialist, having played 409 games for 18 different teams in six different countries across a globetrotting career.
He represented Australia 43 times in white-ball cricket and has won three BBL titles - one each with the Brisbane Heat, Melbourne Renegades and Sixers.
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