The 22 year-old former Everton man was released by Yeovil last season and scored in the Millers win over Doncaster on Tuesday night.
Agard, who can play wide right or as a central striker, had returned to the club on trial after initial talks to sign him a few weeks ago had broken down due to his agents demands.
He becomes manager Steve Evans tenth summer addition and goes into the squad to face Hearts tomorrow..
Evans explained to the clubs official site: "The lad has frightening pace, a real desire to work hard and I think our supporters have already taken to him. He never lets defenders have a minutes rest and is always on the move so defenders find it difficult to mark him.
''Last season he scored almost one goal in every two matches he started and that is a ratio that any striker would be pleased with. He is hungry to score goals and is very excited about the direction Rotherham United is taking. He has trained with us for a week and of course played a meaningful part on Tuesday night and from that we can all see he is a talented lad."
MillersMAD wish Kieran every success with the club.