Williams, 32, announced his retirement just three months ago after Wales lost to Fiji and were knocked out in the group stages of the World Cup.
Only a week ago, Williams was quoted as saying he had no desire to add to his 76 Test caps and return to the international arena.
But new coach Warren Gatland has persuaded the Cardiff Blues flanker to perform a surprise u-turn - and he has today been included in a 28-man Wales squad, to be captained by Ryan Jones.
Williams' inclusion highlights the paucity of open-side flankers available to Gatland.
Robin Sowden-Taylor, 25, is seen as Williams' eventual replacement but cannot currently command a regular place in the Blues side.
Gatland said: "We have had to mix the short-term and the long-term view and Martyn in particular is covering a specialist position.
"I talked to him this weekend and told him we wanted to select him and he took a couple of days to think it over.
"We have some good youngsters coming through, but the likes of Robin Sowden-Taylor are only going to benefit from having Martyn around at the moment.
"We feel he still has a lot to offer and we are delighted he has accepted the invitation to play for his country again."
In the end, Williams was only retired for one game - Wales' 34-12 November defeat to world champions South Africa at the Millennium Stadium.
Wales squad for RBS 6 Nations Championship:
Backs: Lee Byrne (Ospreys), Jamie Roberts (Cardiff Blues), Tom James (Cardiff Blues), Mark Jones (Llanelli Scarlets), Shane Williams (Ospreys), Tom Shanklin (Cardiff Blues), Sonny Parker (Ospreys), Gavin Henson (Ospreys), James Hook (Ospreys), Stephen Jones (Llanelli Scarlets), Dwayne Peel (Llanelli Scarlets), Mike Phillips (Ospreys), Gareth Cooper (Gloucester).
Forwards: Gethin Jenkins (Cardiff Blues), Adam Jones (Ospreys), Duncan Jones (Ospreys), Rhys Thomas (Newport Gwent Dragons), Huw Bennett (Ospreys), Matthew Rees (Llanelli Scarlets), Ian Evans (Ospreys), Ian Gough (Ospreys), Alun-Wyn Jones (Ospreys), Jonathan Thomas (Ospreys), Martyn Williams (Cardiff Blues), Robin Sowden-Taylor (Cardiff Blues), Gareth Delve (Gloucester), Alix Popham (Llanelli Scarlets), Ryan Jones (Ospreys, capt).