The former Barnet midfielder, 26, is eligible to return to action after serving out a two-year ban for a failed drugs test in 2012.
Marshall had been training with the Sky Blues before being offered a temporary contract.
"From the period he has been training with the club he has very much impressed us, not only with his ability but his hunger as well," manager Steven Pressley told the League One club's official website.
"He has had a lot of interest from other clubs so we are delighted that he has chosen to sign with us until the end of the season as it will give us a chance to fully assess him."
Coventry are back in League One action on Tuesday night when they face Bristol City at Ashton Gate.