<jats:p>We have constructed a regional tree ring cellulose oxygen isotope (δ&lt;sup&gt;18&lt;/sup&gt;O) record for the northern Indian sub-continent based on two new records from north India and central Nepal and three published records from Northwest India, western Nepal and Bhutan. The record spans the interval from 1743&amp;amp;ndash;2008 CE. Correlation analysis reveals that the record is significantly negatively correlated with the three regional climatic indices: All India Rainfall (&lt;i&gt;r&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;&amp;amp;minus;0.5, &lt;i&gt;p&lt;/i&gt;&amp;amp;thinsp;&lt;&amp;amp;thinsp;0.001, &lt;i&gt;n&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;138), Indian monsoon index (&lt;i&gt;r&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;&amp;amp;minus;0.45, &lt;i&gt;p&lt;/i&gt;&amp;amp;thinsp;&lt;&amp;amp;thinsp;0.001, &lt;i&gt;n&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;51) and the intensity of monsoonal circulation (&lt;i&gt;r&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;&amp;amp;minus;0.42, &lt;i&gt;p&lt;/i&gt;&amp;amp;thinsp;&lt;&amp;amp;thinsp;0.001, &lt;i&gt;n&lt;/i&gt;&amp;amp;thinsp;=&amp;amp;thinsp;51). The close relationship between tree ring cellulose δ&lt;sup&gt;18&lt;/sup&gt;O and the Indian summer monsoon (ISM) can be explained by oxygen isotope fractionation mechanisms. Our results indicate that the regional tree ring cellulose δ&lt;sup&gt;18&lt;/sup&gt;O record is suitable for reconstructing high-resolution changes in the ISM. The record exhibits significant inter-annual and centennial variations. Inter-annual changes are closely related to the El Niño-Southern Oscillation (ENSO), which indicates that the ISM was affected by ENSO in the past. However, the ISM-ENSO relationship was not consistent over time. Centennial changes in the regional tree ring δ&lt;sup&gt;18&lt;/sup&gt;O record indicate a trend of weakened ISM intensity since 1820. Decreasing ISM activity is also observed in various high-resolution ISM records from southwest China and Southeast Asia, and may be the result of reduced land-ocean thermal contrasts since 1820 CE.
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