Iran Khorramdin, 74, and Akbar Khorramdin, 81, told police they had used sleeping tablets to knock their son out before suffocating and stabbing him, and cutting up his body, the Hamshahri newspaper reported.
The case has stunned Iran, where it has been splashed across the front pages of newspapers with headlines including “Society in shock” and “Occupants of terror house”.
Murder is usually a capital offence in Iran but people who kill their children face a maximum of 10 years in jail.
Khorramdin’s parents could still face the death penalty, however, if they are put on trial for the murder of their son-in-law.