The 27-year-old defender, who has played for his country 23 times, has been without a club since his summer release from Preston and joins the Millers on a deal until the end of January.
"Craig Morgan is a player who I have watched often during the last few years and he is a top-class performer," Millers boss Steve Evans said.
"The lad is comfortable in central defence and at left-back, something I have been looking at for several weeks. At Peterborough he was simply outstanding and a reason why Darren Ferguson got Preston to pay £400,000.
"He is very comfortable on the ball and is a real competitor. Our squad is taking shape very nicely and competition for a places is getting to a level where we need it to be."
Morgan will be included in the squad for Tuesday's JPT clash with York.
- Craig Morgan