Heatwaves are expected to become more frequent, reach higher maximum temperatures, last longer and occur earlier during the grape growing season. Such heat events during berry development are already known to dramatically reduce berry growth and quality by affecting berry size and primary metabolites (e.g. sugars, organic acids, and amino acids). In this study, the effect of three days of exposure to extreme high temperature after fruit-set was investigated with pot-grown fruiting grapevines (Vitis vinifera L. cv. Shiraz). Using a factorial design, heat treatments were applied for three days (+8 °C) and/or three nights (+6 °C), 20 days after the start of flowering, when tannin biosynthesis is at its maximum. Berry growth responses were recorded, and detailed flavan-3-ol and tannin composition determined for individual berry tissues to elucidate the sensitivity of these compounds to high temperature. Locally heated bunches reached a maximum close to 45 °C on the first day of the heating treatment and berry growth and compositional parameters were immediately and substantially affected by high day temperature. In particular, a significant increase in galloylated skin flavan-3-ol subunits and consequently, percentage of galloylation of skin tannins, were observed two weeks after the end of the treatment. However, total skin tannins and size were not affected and no differences in composition were found closer to véraison. A considerable decrease in total seed tannins and changes in other seed compositional parameters were associated with disruption of berry and seed development upon day heating. In the pulp, various compounds (from the myo-inositol pathway, amino acids) were significantly increased under high day temperature while high night temperature reduced malic acid accumulation. Berries heated day and night exhibited the most differences but this was mainly driven by high day temperature.