Since then the 6'3" midfielder has started five of his six appearances to date with Solomons citing his dedication and potential to improve as key to him earning the extension.
He said: "Joaquin [Dominguez] has settled really well in the city and at the club.
"He's a big, powerful player with good skills and offers a lot of value to the depth of our squad.
"He's improved imeasurably over the past four months and we expect that to continue as he applies himself as a full-time professional here at Murrayfield."
Dominguez said: "It's great news. I'm very happy.
"I came to this club looking to make the most of this opportunity and stay long term so its great to have achieved that goal.
"Since I started it's been examination after examination to prove myself to the coaches so I'm delighted to have secured my future here.
"The coaching and conditioning infrastructure is excellent and I think I'll go on to play my best rugby at this club."
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