The 22-year-old requires international clearance before he can be named in Hibs' squad to face Molde in the Europa League on Thursday.
Hyndman, also linked with a short-term switch to Wigan Athletic this window, has previous experience north of the border with Rangers.
Gers supporters voted the United States international their Young Player of the Year after he impressed during the second half of 2016-17.
Speaking to his new side's official website, Hyndman said: "I am delighted to have joined Hibs. I enjoyed my previous time in Scotland - my game is well suited to the style of play here.
"I'm looking forward to meeting up with the squad and hopefully working my way into the team."
Hibs boss Neil Lennon added: "We are pleased to bring in a player of Emerson's quality. Technically he's at a good level, he's got experience of the Scottish game and gives us another option in the centre of midfield."
Hibernian beat Motherwell 3-0 at Easter Road in their opening league game of the season last weekend.