Born in Nelspruit, South Africa, 27-year-old Pretorius qualified to play for Wales on residency last month.
Meanwhile, full-back Lee Byrne has been recalled thanks to his impressive form for Clermont while props Adam Jones and Craig Mitchell have both returned from injury and Ryan Jones is also selected despite dislocating his thumb on Sunday in Ospreys' draw with Leicester.
King, Coombs, a former Wales Under-20 captain, and Pretorius have all been selected in the wake of numerous injury problems for interim coach Rob Howley in the pack.
Alun Wyn Jones, Luke Charteris, Dan Lydiate and Bradley Davies are injured, although all except Charteris, who is out for the season, could return for the latter stages of the competition.
"We have put together a very experienced squad and complimented that by rewarding the in-form players with a call-up," said Howley.
"The players in the squad have been playing really well and we have been pleased with their individual performances.
"We have a group of players who know how to win big matches and have experienced success. We are adding to that, players who have stood out and performed well for the regions this season."
Sam Warburton will again skipper the side which kicks off its campaign against Ireland in Cardiff on February 2.
As well as injury problems in the second and back row, Wales are also without prop Aaron Jarvis, hooker Huw Bennett, centre Ashley Beck and fly-half Rhys Priestland.
FORWARDS: Scott Andrews (Blues), Craig Mitchell (Exeter Chiefs), Adam Jones (Ospreys) Paul James (Bath), Gethin Jenkins (Toulon), Ryan Bevington (Ospreys), Richard Hibbard (Ospreys), Ken Owens (Scarlets), Matthew Rees (Scarlets), Ryan Jones (Ospreys) Lou Reed (Blues), Ian Evans (Ospreys), James King (Ospreys), Andrew Coombs (Dragons), Josh Turnbull (Scarlets), Josh Navidi (Blues), Aaron Shingler (Scarlets), Justin Tipuric (Ospreys), Sam Warburton (Scarlets), Toby Faletau (Dragons), Andries Pretorius (Blues)
BACKS: Tavis Knoyle (Scarlets), Mike Phillips (Bayonne), Lloyd Williams (Blues), Dan Biggar (Ospreys), James Hook (Perpignan), Jonathan Davies (Scarlets), Jamie Roberts (Blues), Scott Williams (Scarlets) Alex Cuthbert (Blues), George North (Scarlets), Eli Walker (Ospreys), Leigh Halfpenny (Blues), Liam Williams (Scarlets), Lee Byrne (Clermont)