The forward spent nine years with Dundee United before leaving Tannadice when his contract ended in the summer.
Dow, 25, who made 21 appearances for United last season, has been training at Dingwall for the past three weeks.
"Ryan has been with us for the last few weeks while we assessed his fitness and we are delighted he has agreed to join," said Ross County manager Jim McIntyre.
"Ryan has fitted in really well and we think we are getting a great player who will add to our already strong attacking options."
Ryan Dow has signed a deal with Ross County until the end of the season.