Leacock last played on the final day of last season against Reading
Leyton Orient have signed former Derby County defender Dean Leacock until the end of the season.
The 27-year-old, who plays centre-back or right-back, joined the Rams in August 2006 and helped them back to the Premier League in his first term.
He made 127 appearances for Derby after starting his career at Fulham and spending two loans at Coventry.
The O's have also signed Tunisian centre-back Syam Ben Youssef, 22, for the remainder of this season.
Youssef won his country's domestic league with Tunis-based side Esperance during his two seasons at the club, which he left last summer.
Leacock, who cancelled his Derby contract five months early, has not made an appearance for the Rams this season and signed too late for Orient to feature on Saturday.
Manager Russell Slade said: "We're delighted to add Dean to the squad. He has a fine pedigree having played all his career in the Premier League and the Championship.
"We've had to bide our time and work hard to get our man and we're looking forward to seeing him in red and white."