Lions hooker Matthew Rees captains the side for a ninth time, while Ospreys centre Andrew Bishop and fly-half Dan Biggar make their first appearances since before the 2011 World Cup.
Caretaker coach Rob Howley, who is standing in for Warren Gatland after the New Zealander broke both heels in a fall in April, has picked just four Lions in his starting side and one more on the bench as 16 of his first-choice team are heading down to Australia tomorrow to prepare for the three-Test series with the Wallabies.
Alun Wyn Jones starts in the second row as he won't be travelling to Australia until after his sister's wedding on June 1, while fellow Lions James Hook and Ryan Jones join Rees in the starting XV, with Williams in line to become a centurion from the bench.
Scarlets centre Adam Warren could take the number of debutants to four after being selected among the replacements, while clubmate Josh Turnbull gets a chance to impress from the blindside flank.
"We have picked a side on form, with an eye on bringing players through for the future, but very much with the intention of getting the summer campaign off to a winning start," said Howley.
"We have momentum from the RBS 6 Nations and the Ospreys have added to that with a great win in our domestic competition. The Ospreys have been in a rich vein of form and their victory over Leinster last weekend to take the RaboDirect PRO12 title was a boost for the whole of Welsh rugby.
"Dan (Biggar) and Andrew (Bishop) have been playing particularly well and we are looking forward to seeing both players in a Welsh shirt again.
"We are now looking for the next step and a victory on home soil against a formidable Barbarians team before we set off for Australia."
The final members of Wales' tour party, which will total 34 players, will be named at the Millennium Stadium on Saturday immediately after the Baa-Baas fixture, with the team to play the Wallabies in the first Test in Brisbane on June 9 due to be named the following Thursday.
Wales: Liam Williams (Scarlets); Harry Robinson (Blues), Andrew Bishop (Ospreys), James Hook (Perpignan), Aled Brew (Dragons); Dan Biggar (Ospreys), Lloyd Williams (Blues); Rhys Gill (Saracens), Matthew Rees (Scarlets) Captain, Rhodri Jones (Scarlets), Alun Wyn Jones (Ospreys), Ian Evans (Ospreys), Josh Turnbull (Scarlets), Justin Tipuric (Ospreys), Ryan Jones (Ospreys)
Replacements: Richard Hibbard (Ospreys), Paul James (Ospreys), Aaron Shingler (Scarlets), Martyn Williams (Blues), Rhys Webb (Ospreys), Adam Warren (Scarlets), Will Harries (Dragons)
NB Bold denotes Lion