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This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]