The 24-year-old has made 14 international appearances in the last three years and is likely to be an important member of Italy's World Cup squad as the anointed successor to Italian legend Diego Dominguez.
He will join up with the Tigers after the tournament and Director of Rugby Dean Richards is looking forward to seeing him in action at Welford Road.
'Ramiro is a really exciting player and I'm delighted that he will be joining us. He has a good left foot, but is far more than a kicking fly-half," said Richards.
"He takes the ball up to the gainline extremely well and is just the kind of player who will put opposition defences on the back foot.
"I'm sure he will become a real favourite with our supporters. He is a very likeable young man who is extremely dedicated to training."
Pez said he was delighted to be joining Leicester Tigers after two successful seasons at Rotherham which saw the Yorkshire club secure two titles, if not promotion just yet.
"I'm so happy to be coming to Leicester," he said. "I've had two great years at Rotherham and have made some really good friends there, but this is a brilliant opportunity for me.
"Leicester Tigers are such a good team and when they approached me I was very pleased indeed. The club has had so much success over the last few years and I really want to be part of that. I'm very impressed with what I've seen so far. The facilities are excellent and everybody has made me feel very welcome.
"I can't wait for my first game – it will be a very big moment for me."
This latest signing means that Leicester have found a solution to the problem that has plagued them over recent months of who to field in the No.10 shirt.
Long-term injury victim Austin Healey has made no secret of his desire to appear at scrum-half, while the departure of Rod Kafer and Andy Goode in recent years has further depleted Leiceser's options in this crucial area.
Wearing the No.10 shirt for most of this season was Sam Vesty, with Australian Craig McMullen deputising.