Scully, who plays full-back and wing, has been training with Tigers and played in the opening round of the JP Morgan Premiership Sevens series as well as Friday’s opening pre-season fixture against Montpellier.
Tigers director of rugby Richard Cockerill said: “We’ve been looking for a player to cover positions in the back three, we’ve had a look at Blaine over the last couple of weeks and have now offered him a contract for the next four months.
“He has been working hard with us, he has some good skills and he is very keen to learn and develop his game, which is important.”
Scully headed back to the USA following Friday’s match against Montpellier in Geneva as he is included in the national squad for two vital Rugby World Cup qualifying fixtures against neighbours Canada this month. He will return to Tigers before the start of the Aviva Premiership season.
California-born Scully, 25, said: “I came over on trial to try to earn my spot here and I have learned a lot even in the couple of weeks I’ve spent with Leicester.
“This is a big opportunity and I couldn’t ask for a better place to start as a player.
“The attitude of the players is fantastic. They just want to work hard and get better everyday and that is something I really embrace.”