Gallacher was most recently with Scottish Championship rivals St Mirren and started his career at Rangers before having a spell at Hearts.
The 27-year-old, who trained with Hibs on Thursday, is joining the club as cover for first choice Ofir Marciano, who is recovering from injury.
Marciano came off after 20 minutes of Saturday's 8-1 Scottish Cup win over Bonnyrigg Rose.
Ross Laidlaw came on in the holders' Scottish Cup fourth-round win and Hibs, who lead the second tier by six points, visit Queen of the South on Saturday.
Neil Lennon's side will face Edinburgh rivals Hearts in the next round, the Tynecastle side having won their fourth-round replay 4-2 against visitors Raith Rovers on Wednesday.
"It was a good cup tie last night and could have gone either way," said Lennon.
"We know we are playing Hearts now but we will put that to the back burner. It's a big game to look forward to but we have other big games lined up as well and none more so than Queen of the South at the weekend.
"The priority is to get out of this division and we haven't done that for a few seasons. We really want to do that. We have put ourselves in a decent position at the minute.
"The cup will look after itself and if you are going to win the cup you have to beat what's put in front of you. It just happens to be a local derby so we will address that when it comes around."
Kris Commons has returned to Celtic following his recent emergency loan at Hibs but Lennon hopes to bring the 33-year-old forward back to Easter Road.
"I expect it to go to the wire in regards of Kris," added Lennon. "I don't know if he has forced his way back into [Celtic manager] Brendan [Rodgers'] thinking or not.
"He is a good player. We are in the hunt, put it that way.
"A lot of managers' phones will be on until the window closes. A lot can happen between now and next Tuesday."
Hibernian have signed goalkeeper Scott Gallacher until the end of the season.