The 24-year-old has flown out to join up with the Millers at their pre-season training camp in Austria and could play against Floridsdorfer in tomorrow’s friendly.
Moore moved to Portman Road from Forest Green Rovers last January.
He had previously had a two-year spell at Yeovil Town.
Millers’ boss Paul Warne said: “We are delighted to bring Kieffer into the club.
“He played against us last year and did well. We spoke to a few people about him and it was all good news. He has got some amazing attributes. He is really keen and ready to learn. He is very athletic and very strong and it is another striker to add to our attacking force.
“Kieffer’s arrival now gives us more options. I have always said the more strikers and the more competition I can get up front the better.”