The incident took place last evening when the man took the four-month-old daughter of a tea vendor from her unsuspecting family to an isolated place and raped her, Bhojpur SP Satyaveer Singh said.
The man then abandoned the infant and fled the place, from where her worried family members later found her.
The bruised infant was rushed to Sadar hospital from where she was referred to Patna Medical College and Hospital, where a medical examination confirmed rape, the SP said.
An FIR has been lodged against the culprit, who is absconding, and raids are on to catch him, he said.