The 20-year-old, who can operate at centre-back, right-back, and wing-back, lined up for the Gunners in their pre-season friendly defeat at Easter Road before joining Ross County on a season-long loan.
Clarke made 17 appearances for the Staggies, scoring three goals and laying on two more, but was recalled by the English side in early January.
Manager Shaun Maloney said: “Harry has very good technical attributes in both attacking and defensive actions, and I’m really pleased we’ve been able to secure him for the club.
“He will bring an exciting energy and speed in both areas, and I have no doubt he’ll be a big asset for us going forward.
“I know how hungry he is to succeed and be a part of this club and over the coming days I’m very much looking forward to working with him.”
Hibs have been in need of defensive reinforcements since last summer and even though Nathan Wood arrived on a season-long loan from Middlesbrough, he returned to Boro in December having made just one appearance.
With Ryan Porteous missing three matches this month and Paul McGinn at risk of suspension when he faces Hampden top brass at an SFA hearing into his comments after the Premier Sports Cup final, Clarke could make his debut when Hibs travel to Celtic on January 17.