Vallejos has been with Pumas in recent weeks as part of the inaugural Southern Hemisphere rugby championship contested by elite teams including the All Blacks, South Africa and Australia.
Born in Rosario, Vallejos has represented Argentina in the U17, U18, U20 and U23 age groups as well as achieving his first cap for the Pumas after featuring for his country in the 2011 Rugby World Cup.
The second row had been with Harlequins for the last two years and has impressed with his athleticism and skill at set piece. Prior to his time with the English club he played with Parma in Italy.
Keen to be part of his new set up in West Wales, Vallejos packed his bags immediately after the last international game at the weekend against Australia in his home-town of Rosario and he and his girlfriend Lucia jumped on a plane from Buenos Aires to London.
Arriving at Parc y Scarlets, Tomás Vallejos said: "I just wanted to get here as soon as possible and be part of Scarlets training this week as we're going into Europe.
"It's a very exciting time for me to join and such a great opportunity to be part of a club with real history and with the same passion and pride in rugby that I recognise and understand from my own background.
"I first saw the Scarlets set-up when I met with Nigel Davies here in March and was really impressed with everything I saw and heard, the facilities here are so impressive and gives you all you need as a player - but it was also about where this club has come from and what it stands for.
"I was really taken by how strongly people feel about rugby here. You can tell how much they value the sport and work so hard to develop and support it and are so obviously passionate about what they do and how club works with its community.
"That's something important to me as a individual and I recognise it from home. When I saw the sign across the stand 'heart and soul rugby country' - it just struck me and it's those little details that make you want to be part of a place."
Vallejos joins a strengthened Scarlets pack which provides plenty of competition in the second row with new signings South Africans George Earle and Joe Snyman, Australian Jake Ball as well as Sione Timani, Richard Kelly and young prospect Joel Galley all in the running for places this season.
Tomás Vallejos, who celebrates his 28th birthday in West Wales on the 16th of October said he was eager to get into training at Parc y Scarlets; "It was good experience spending time in the international set-up but it was a bit tough not getting as much game time in the Argentina shirt as I had hoped and the opportunity to show what I can do.
"Simon Easterby was good enough to say I could take a couple of days break before I travelled to Wales, but I just wanted to get here and it's great to be in with the squad now. I have watched the highlights of the Scarlets games in the league and they are playing some great rugby.
"My former coaches have said they think I will fit in here well as I play and enjoy the same type of rugby that the Scarlets are known for.
"It's a very positive time to be joining, there's plenty of competition and it's been a strong start to the season. It's clear Simon Easterby and his team are ambitious and focused - I'm looking forward to working with him and alongside the Scarlets players; and from my first impressions it looks a great place to be a part of."