Hook will start on the bench at the Millennium Stadium and provide cover for a starting backline featuring three enforced changes from the dramatic 28-25 victory over England last weekend.
Matthew Morgan replaces Liam Williams at fullback, Tyler Morgan comes in at outside centre in place of Scott Williams, while Alex Cuthbert starts out wide instead of Hallam Amos with George North switching to the other wing.
Scott Williams and Amos were released from the squad on Monday because of knee and shoulder injuries sustained against England.
Liam Williams remains in the squad but was not considered for the Fiji match after suffering a concussion in last Saturday's match at Twickenham.
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They were just the latest injury blows for Wales, who lost first choice centre Jonathan Davies, fullback Leigh Halfpenny and scrumhalf Rhys Webb before the start of the tournament.
Gatland was, however, able to name an unchanged pack for what should be a tough battle up front against a Fiji side who have been impressive in defeats to England and Australia.
The coach said he was delighted to be returning to Cardiff, where the Welsh opened their campaign with a 54-9 victory over pool minnows Uruguay.
"It is important we build on the success and momentum from last weekend and take that into Thursday," Gatland said in a news release.
"Fiji have performed very well in their opening two games and they will be coming to Cardiff looking to build on that themselves.
"We have made three changes through injury but go into this game with a strong side. We are looking forward to being back at the Millennium Stadium on Thursday and having the crowd right behind us in an important World Cup fixture."Team: 1-Gethin Jenkins, 2-Scott Baldwin, 3-Tom Francis, 4-Bradley Davies, 5-Alun Wyn Jones, 6-Dan Lydiate, 7-Sam Warburton (captain), 8-Toby Faletau; 9-Gareth Davies, 10-Dan Biggar, 11-George North, 12-Jamie Roberts, 13-Tyler Morgan, 14-Alex Cuthbert, 15-Matthew Morgan
Replacements: 16-Ken Owens, 17-Aaron Jarvis, 18-Samson Lee, 19-Luke Charteris, 20-Justin Tipuric, 21-Lloyd Williams, 22-Rhys Priestland, 23-James Hook
(Reporting by Nick Mulvenney in Nottingham, editing by Sudipto Ganguly)
