The 24-year-old joined Mick McCarthy's side in January but played just 146 minutes in the league for the Tractor Boys and will now look to get regular game time under his belt with the League One Millers.
"We are delighted to bring Kieffer into the club," Rotherham manager Paul Warne told the club's website. "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."
Moore has flown out to Austria to join his new teammates at their pre-season training camp and is expected to make his debut in a friendly against Floridsorfar on Tuesday night.