The former IRB World Player of the Year, who won 87 caps for Wales between 2000 and 2011 and toured twice with the Lions, is one of three Welshmen named in John Kirwan's starting side.
Williams is picked on the left wing, with Richie Rees chosen at scrum-half and Duncan Jones packing down at loose-head prop.
Rees has won nine caps for Wales so far and will see Saturday as a good chance to put pressure on the likes of Mike Phillips, Lloyd Williams, Rhys Webb and Tavis Knoyle ahead of his move from the Blues to Edinburgh.
Jones won the first of his 57 international caps in 2001 but hasn't featured for Wales since November 2009 when he came off the bench in defeat to Australia.
Blues centre Casey Laulala will play his last game in the capital before heading to Munster, while fellow All Blacks Mils Muliana and Stephen Donald start at full back and fly-half respectively for the invitational outfit.
Leinster's Fijian wing Isa Nacewa will be hoping for a repeat of his late match-winning score in last year's fixture as he completes a backline that also includes England's World Cup winning centre Mike Tindall.
Former South Africa skipper John Smit captains the side from tight-head, while Bath flanker Francois Louw is the second Springbok in the starting side.
Second row Mick O'Driscoll and flanker Johnnie Beattie provide Irish and Scottish representation, with Benoit August of France, Mark Chisholm of Australia and Mamuka Gorgodze of Georgia taking the total number of countries represented in the first XV to 10.
Barbarians: M Muliana; I Nacewa, M Tindall, C Laulala, S Williams; S Donald, R Rees; D Jones, B August, J Smit (captain), M O'Driscoll, M Chisholm, F Louw, J Beattie, M Gorgodze
Replacements: A de Malmanche, N Tialata, A Van Zyl, A Qera, R Lawson, S Tagicakibau, C Heymans