That means Cardiff Blues 10 Jarrod Evans, who started Saturday’s warm-up loss to Ireland, misses out.
Lock Cory Hill is included despite currently being sidelined with a small fracture in his leg.
There is no place for Scarlets props Samson Lee and Rob Evans, or centre Scott Williams.
Evans was first choice during Wales’ 2019 Grand Slam campaign but Saracens’ Rhys Carre makes the plane to Japan as one of five props after making his debut against the Irish.
Nicky Smith, Tomas Francis, Dillon Lewis and Wyn Jones are the other props. Jones will cover both sides of the scrum with only two tight-heads selected.
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Ken Owens, who is heading to his third World Cup, Elliot Dee and Ryan Elias are the three hookers in the squad.
“Selection is always the toughest part of the job and that is especially true come RWC time,” said Gatland.
“Reducing the squad down to 31 has been extremely hard especially when you look at the depth we have created and the amount of work the training squad have put in.”
Hill is not expected to be available for the start of Wales’ campaign, which begins on 23 September against Georgia.
But the 27-year-old gets the nod in the second-row alongside captain Alun Wyn Jones, Jake Ball and Adam Beard.
Wales have named six back-rows in James Davies, Ross Moriarty, Josh Navidi, Aaron Shingler, Justin Tipuric and Aaron Wainwright.
Gatland has chosen 18 forwards and 13 backs in his 31-man squad.
Gareth Davies, Aled Davies and Tomos Williams are named as the scrum-halves.
Centre Owen Watkin has been given the nod ahead of Ospreys team-mate Scott Williams, while Hallam Amos is selected in the back three ahead of Owen Lane who scored on his debut in the 22-17 defeat against Ireland.
Jonathan Davies, Hadleigh Parkes and Watkin are the centres in the squad, with Josh Adams, Amos, Leigh Halfpenny, George North and Liam Williams included as back-three selections.
The nine players who have missed out on World Cup selection are Jonah Holmes, Lane, Steff Evans, Scott Williams, Jarrod Evans, Rob Evans, Lee, Leon Brown and Bradley Davies.
“We are really happy with the final 31, we feel there is an excellent blend to the squad, in terms of talent, experience and age profile and we are all incredibly excited about heading to Japan and what lies ahead,” Gatland added.
“These players have performed and delivered for Wales and deserve the opportunity to represent their nation at the game’s showpiece tournament.”
What started as an extended training group of 42 players had already been reduced by two tournament-ending injuries to number eight Taulupe Faletau and fly-half Gareth Anscombe.
Wales begin their World Cup campaign in Japan against Georgia on Monday, 23 September in Toyota City before further pool matches against Australia, Fiji and Uruguay.
Wales World Cup squad
Props: Nicky Smith (Ospreys), Wyn Jones (Scarlets), Rhys Carre (Saracens), Tomas Francis (Exeter Chiefs), Dillon Lewis (Cardiff Blues).
Hookers: Ken Owens (Scarlets), Elliot Dee (Dragons), Ryan Elias (Scarlets).
Second-rows: Alun Wyn Jones (Ospreys, capt), Adam Beard (Ospreys), Cory Hill (Dragons), Jake Ball (Scarlets).
Back-rowers: Aaron Shingler (Scarlets), Josh Navidi (Cardiff Blues), Ross Moriarty (Dragons), Aaron Wainwright (Dragons), Justin Tipuric (Ospreys), James Davies (Scarlets).
Scrum-halves: Gareth Davies (Scarlets), Tomos Williams (Cardiff Blues), Aled Davies (Ospreys).
Fly-halves: Dan Biggar (Northampton), Rhys Patchell (Scarlets).
Centres: Hadleigh Parkes (Scarlets), Jonathan Davies (Scarlets), Owen Watkin (Ospreys).
Back three: Liam Williams (Saracens), Leigh Halfpenny (Scarlets), George North (Ospreys), Hallam Amos (Cardiff Blues), Josh Adams (Cardiff Blues.