Koroma came through Orient’s academy, made his first-team debut aged 17 and helped the east Londoners win the National League in 2019.
His displays that season, which included 11 goals in 39 league appearances, also earned him a move to then-Championship side Huddersfield.
Koroma scored 35 goals in 168 appearances for Huddersfield in a six-year stay, that also included loan spells at Rotherham United and Portsmouth.
Orient head coach Richie Wellens said: “I am so pleased to get someone of Josh’s quality through the door.
“He needs no introduction to our fans and returns to us as a man and someone that now has great experience at this level and the level above.
“He should be coming into the prime years of his career and if we can get the best out of him then he can be a special player for us.”