A self-closing device is for sliding doors of the type including at least one sliding leaf (2) which is hung from an upper profile (3). The device (1) includes a drive belt (10) configured to be joined to the sliding leaf (2); a closing pulley (20), provided with an elastic device (30), which is arranged on a first rotation axis (4) integral to the upper profile (3) and which engages with a first end (11) of the drive belt (10); and a brake pulley (40), provided with a stop (50), which is arranged on a second rotation axis (5) integral to the upper profile (3) and which engages with a second end (12) of the drive belt (10) opposite to the first end (11). The rotation movement of both pulleys (20, 40) is connected by the drive belt (10).