Police detained a man after an allegation that a fifth-grade student was sexually assaulted in the Singinala area of Khagrachhari Sadar.
The alleged incident took place at about 11:30am on 4 August, while the child was on her way to school, her family said.
Her family alleged that the man took her to a nearby wooded area and assaulted her.
The child later told a classmate and a teacher about the allegation, family members said.
Residents then held the suspect until Khagrachhari Sadar police took him into custody.
Khagrachhari Sadar police said legal procedures were under way and that the child would be sent for a medical examination.
Police said a case had been filed and that the investigation would consider medical findings, witness testimony and other evidence.
No medical examination result, charge decision or court outcome was available.