IIE, the interface between the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM), is defined using the equivalent potential temperature and summer long‐term mean reanalysis data provided by NOAA/OAR/ESRL PSD. The June–July–August reanalysis data for the period 1951–2008 and empirical orthogonal function analysis are further applied to obtain the IIE index at the near‐surface isobaric level. The index has a prominent interannual variation that is strongly correlated with the seesaw variation between the ISM and EASM. When a strong EASM and weak ISM occur, this interface index is higher than the normal, with the interface between the two summer monsoons shifting farther eastward than normal. When a weak EASM and strong ISM appear, the index is lower than normal, with the interface moving farther westward than normal. The western North Pacific subtropical high, a major factor in the EASM system, plays an important role in the year‐to‐year variation of the IIE. Compared with approaches taken in previous studies, this index objectively and quantitatively describes the IIE variation and better represents the two teleconnection patterns associated with the Asian summer monsoon, thus enhancing interpretations of the interaction between the ISM and EASM and its effects on regional droughts and floods in East Asia.