The weapon had fallen into cushions as a shopper sat down to test the sofa at the store in a suburb of the state capital, Indianapolis.
Nobody was hurt in Monday's incident and no arrests have been made. Ikea said it had expressed "sincerest apologies" to the child's family.
Police say the gun-owner did not notice he had dropped his firearm.
"He sat down and somehow or another it became dislodged from this body and when he got up he didn't realise that he was without it," Tom Weger of the Fishers Police Department told the Washington Post.