Williams will win his 99th cap, taking him a step closer to the all time appearance total record of 100, held jointly by Gareth Thomas and Stephen Jones, although Jones could steal the limelight if he appears from the bench to break clear with 101 caps.
Both Sam Warburton (Blues) and Dragons No8 Toby Faletau make way but Dan Lydiate (also Dragons) retains his place on the flank to complete the back row unit.
British & Irish Lions prop Adam Jones adds his considerable experience (69 caps) to the front row alongside fellow Osprey Richard Hibbard, who completes his return from shoulder surgery to be preferred to Dragon Lloyd Burns, who missed out alongside prop Craig Mitchell (Exeter Chiefs).
Bradley Davies (Blues) returns in the second row to be re-united with Ospreys lock Alun Wyn Jones, with Dragon Luke Charteris omitted.
In the backs there are four changes of personnel with Ospreys full back Lee Byrne and Blues wing Leigh Halfpenny both returning to full fitness.
Jonathan Davies (Scarlets) replaces the injured Gavin Henson and is re-united with Jamie Roberts (Blues) in the centre, although it is Roberts who will fill the inside centre berth.
Scarlets scrum half Tavis Knoyle is handed his first start of the summer warm-up matches ahead of Osprey Mike Phillips, with Lloyd Williams (Blues) providing specialist cover from the bench and set to win his first cap.
Elsewhere, George North (Scarlets) switches wings to accommodate Halfpenny with Osprey Shane Williams the one to miss out and James Hook (also Ospreys) takes over at fly half replacing the injured Scarlet Rhys Priestland, after starting at full back last weekend.
On the bench Jones and the uncapped Williams are joined by Dragons wing Aled Brew in providing cover for the backs and up front Ospreys pair Ryan Bevington (prop) and Huw Bennett (hooker) are retained.
Uncapped Ospreys flanker Justin Tipuric has another chance to make his Millennium Stadium debut and Andy Powell (Sale Sharks) will hope to make an impact on the coach's RWC selection plans if called upon.
"Criteria for selection has been about putting together a side to win the game, but also about giving some players the opportunity to impress and, therefore, bid for selection for the world Cup," said head coach Warren Gatland.
"In some areas we are also looking at combinations with that opening game against South Africa on 11 September always kept in mind.
"Beating the Pumas on Saturday would also mean we retain that all important winning momentum going into the World Cup.
"But there are still many places up for grabs on that plane to New Zealand and the players all know they need to perform against the Pumas in order to further their own as well as the national cause."
Walesassistant coach (backs) Rob Howley added: "We are particularly pleased to get the chance to see Leigh Halfpenny back in a Wales shirt after such a long lay-off, he is looking good at the moment, as is Lee Byrne.
"We have selected an exciting back three and there are one or two combinations in the back line which we wanted to have a look at.
"Scott Williams has done a fine job when called upon and is, perhaps, unlucky not to feature but fielding the established partnership of Jamie and Jonathan in the centre also gives us the continuity we need.
"James gets the chance to show what he can do once again when he starts at fly half and Tavis also has an opportunity to put his hand up.
"In terms of scrum halves there is also a young man on the bench, who people may not know too much about, but who has impressed us, in Lloyd Williams and we hope to see him at some stage as well."
Blues prop Gethin Jenkins remains unavailable for selection this week due to a calf problem and there are a number of players carrying bumps and bruises from the match last weekend who have taken a limited part in training during the early part of this week in Luke Charteris, Craig Mitchell, Scott Williams and Aled Brew.
WALES:Lee Byrne; Leigh Halfpenny, Jonathan Davies, Jamie Roberts, George North; James Hook, Tavis Knoyle; Paul James, Richard Hibbard, Adam Jones, Bradley Davies, Alun Wyn Jones, Dan Lydiate, Martyn Williams, Ryan Jones (captain)
REPLACEMENTS: Huw Bennett, Ryan Bevington, Andy Powell, Justin Tipuric, Lloyd Williams, Stephen Jones, Aled Brew