The 30-year-old has been with the Dingwall club since leaving Partick Thistle in 2008.
But he has not played at all this season and has only made three appearances since December.
Boyd, who started his career with Livingston, becomes Killie boss Lee Clark's 16th signing this summer.
Kilmarnock have signed defender Scott Boyd on a season-long loan from Scottish Premiership rivals Ross County.