Williams ended his Wales career in spectacular fashion last December, as he scored a last minute try in the slender 24-18 loss to Australia. But he will have one final swansong at the home of Welsh rugby.
Centurion Stephen Jones has also been included in a strong Baa-Baas squad. The Wasps-bound fly-half was left out of Wales' 38-man training squad yesterday as the management focus on the future.
Ospreys prop Duncan Jones completes a trio of Welshmen in the squad and could make his first Millennium Stadium appearance since 2009.
Martyn Williams is also set to win his 100th Wales cap against his former international team-mates.
The famous invitational club boast the likes of Isa Nacewa, Mils Muliaina, Cedric Heymans, Stephen Donald and John Smit in their star-studded squad.
The Baa-Baas face England and Ireland ahead of the June 2, clash with Rob Howley's Wales.
Backs: Isa Nacewa (Fiji), Mils Muliaina (New Zealand), Ian Balshaw (England), Cedric Heymans (France), Shane Williams (Wales), Paul Sackey (England), Sailosi Tagicakibau (Samoa), Eliota Fuimaono-Sapolu (Samoa), Mike Tindall (England), Damien Traille (France), Felipe Contepomi (Argentine), Stephen Donald (New Zealand), Stephen Jones (Wales), Rory Lawson (Scotland), Jerome Fillol (France)
Forwards: Schalk Brits (South Africa), Benoit August (France), John Smit (South Africa), Aled de Malmanche (New Zealand), John Afoa (New Zealand), Duncan Jones (Wales), B J Botha (South Africa), Neemia Tialata (New Zealand), Jerome Thion (France), Mick O'Driscoll (Ireland), Anton van Zyl (South Africa), Pelu Taele (Samoa), Raphael Lakafia (France) Antoine Burban (France), Akupusi Qera (Fiji), John Beattie (Scotland), Pedrie Wannenburg (South Africa), Juan Manuel Leguizamon (Argentine)