Current climate conditions indicate that California’s viticultural regions will experiencemore frequent heat events, thus leading to greater water stress resulting in lower quality fruit in grapevines. One strategy used to help mitigate the negative effects is irrigating throughout these events to help preserve berry composition. In order to evaluate the usefulness of irrigation throughout heat events, Vitis vinifera cv. Cabernet Sauvignon vines in an established vineyard were exposed to three differential irrigation treatments. In 2020, the baseline treatment was under water deficit (60% ET), while the 2x baseline (120% ET) and 3x baseline (180% ET) treatments had double and triple the irrigation of the baseline, respectively. The irrigation treatments were adjusted in 2021 to see if less water could be used without negatively impacting the phenolics results. The treatments were 60% ET, 90% ET, and 120% ET. Differential irrigation started one to two days prior to a heat wave (HW) and continued until the last day of the HW. In this study, a HW was defined as three consecutive days with maximum temperatures at or above 38°C. Throughout the 2020 and 2021 growing seasons, there were major declines in anthocyanin concentration for the 60% ET treatment, which suggests a suppression of anthocyanin synthesis and promotion of degradation at such high temperatures. A common trend seen throughout both years is that the 180% and 120% treatments from 2020 and 2021 respectively, resulted in lower phenolic concentrations than the 120% and 90% treatments. This is important to note as these results suggest that irrigation prior to heatwaves can be beneficial in maintaining fruit quality, but excessive watering may negate the beneficial aspects of deficit irrigation. Transcriptomic analyses identified the down-regulation of genes involved in the phenylpropanoid and flavonoid pathways, which provides a potential explanation at the molecular level for the lower phenolic concentrations. This study highlights the detrimental effects of too little or excess water application during HWs on grape berry composition and gene expression.