The 25-year-old joins from Championship side Rotherham, for whom he scored 11 tries this season.
Monahan said: "It's a fantastic opportunity for me to pit myself against the very best in the English club game. Gloucester already have some really talented backs in the squad and it's going to be great for my game to work with them. They've also signed some class players for next season so it's a really exciting prospect."
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