Phillips, who retired at the end of last season, has answered an injury SOS from the Llanelli-based side and is set to feature off the bench in Sunday’s Guinness PRO14 clash with Southern Kings.
Now 35, Phillips won two Six Nations titles and one grand slam with Wales and is currently living in Dubai.
He will meet the rest of the Scarlets’ squad in South Africa for the game against the Kings.
“We are delighted Mike has been able to answer the call and will be joining us in South Africa,” said Scarlets head coach Wayne Pivac.
“He has a wealth of experience, is a proud former Scarlet, and will be a great addition to our travelling squad.”
Pivac has seen his scrum-half stocks dwindle with both Gareth Davies and Aled Davies on Under Armour Series duty with Wales this month.
Jonathan Evans remains available for selection, but a broken leg suffered by young No 9 Declan Smith left Pivac without a back-up in that position.
Veteran Phillips, who won 94 caps for Wales and five for the British & Irish Lions on tours to South Africa and Australia, has stepped into the void.
Pivac said: “It’s just been one of those things! We only have one of our four contracted scrum-halves available through injury and international call-ups.
“We looked at the nines available to us within the region and Welsh qualified players in the Championship, but they were unable to travel due to work and club commitments.”
Meanwhile, Scarlets assistant coaches Stephen Jones and Byron Hayward have followed Pivac in signing new deals with the Scarlets.
Attack supremo and former Wales fly-half Jones and defence chief Hayward helped Scarlets to a stunning Guinness PRO12 title success last season.
“I am delighted and honoured to have extended my time at this wonderful region,” said Jones.
“I am fortunate to be working with a fantastic group of coaches and players and I am looking forward to continuing to build on our success. On a personal level I am looking to continue to improve and develop our attacking game.”
Hayward added: “I feel very fortunate and blessed to be working with the group of staff and players that I’m working with at the Scarlets.
“The culture and environment we’ve developed over the last few years is very enjoyable and that was a fundamental part of me wanting to continue my time here at the Scarlets.
“We’re continuously pushing to raise the standards and the players keep challenging us as coaches. Every year we’re reinforcing our defence principles and all of the players fully understand what we’re trying to achieve.
“It’s great to see that they have the mindset of always wanting to evolve and get better.”
