Details of subsurface ocean conditions associated with the Indian Ocean Dipole event (IOD) were observed for the first time by mooring buoys in the eastern equatorial Indian Ocean. Large‐scale sea surface signals in the tropical Indian Ocean associated with the positive IOD started in August 2006, and the anomalous conditions continued until December 2006. Data from the mooring buoys, however, captured the first appearance of the negative temperature anomaly at the thermocline depth with strong westward current anomalies in May 2006, about three months earlier than the development of the surface signatures. These subsurface evolutions within the ocean would be a key factor for better understanding of IOD mechanisms and its predictability, and are providing a fundamental dataset for validation of modeling outputs.