The 35-year-old openside admits facing his team-mates and playing against the country he loves will be an unusual experience but on he never had any intention of turning down, especially as he believes it gives him a great chance of impressing Wales coach Warren Gatland ahead of the 2011 World Cup.
"It sounds strange to be playing against Wales for once but I'm really looking forward to it," said Williams.
"It's a huge honour to be asked to play for the Barbarians, any player will tell you that, and it's something special to be playing for them against your home country.
"Hopefully, if I can put in a good performance, I can get back into Warren's plans for the World Cup. Having spoken to Warren, I know it's a case of the more Welsh players on the field the better.
"The coaches know what I can and can't do, but it will be a fantastic occasion and I can't wait to run out at the Millennium Stadium against boys I've played with there so many times.
"I would love to go New Zealand, the Rugby World Cup is the pinnacle of anyone's international career, along with the British Lions.
"If I was asked to go, I would grab the opportunity with both hands but there is a lot of rugby to be played before then with the end of the Magners League and the summer internationals. And the one place we have a lot of strength in depth is in the back row."
Wales boss Gatland insists the decision to allow Williams to play against his international team-mates doesn't mean he is now surplus to requirements.
Williams hasn't featured at the Millennium Stadium since the November loss to the All Blacks, with fellow Cardiff Blues star Sam Warburton being preferred in the No7 shirt and the likes of Scarlets duo Rob McCusker and Josh Turnbull gaining preference among the replacements.
"Playing for the Barbarians does not exclude Martyn from the opportunity of still being selected for our World Cup squad," said Gatland.
"With Martyn in the opposition line-up, it's a great chance for us to see how our young back row will cope in an international match against such an experienced and world-class player."