AbstractIn California, over 3.4 million tons of wine grapes were crushed in 2020 while every year roughly 20% of the grape mass goes unused. Grape cluster thinning at veraison, a common agricultural practice to ensure color homogeneity in wine grapes, adds to the production costs and generates substantial on‐farm loss during grapevine cultivation in which the health‐promoting values of thinned clusters (unripe grapes) are usually overlooked. In particular, the health‐promoting properties of flavanol monomers, specifically (+)‐catechin and (−)‐epicatechin, and their oligomeric procyanidins, have been extensively studied in cocoa and chocolate but not so much in grape thinned clusters in recent epidemiology studies. As part of the important agricultural by‐products upcycling effort, the current study compared thinned clusters from Chardonnay and Pinot noir, two premium wine grape varieties cultivated in California, to a traditionally Dutch (alkalized) cocoa powder that has been widely used in food applications. Thinned cluster fractions from Chardonnay and Pinot noir grapes grown in the North Coast of California showed much higher concentrations of flavanol monomers and procyanidins, with 208.8–763.5 times more (+)‐catechin, 3.4–19.4 times more (−)‐epicatechin, and 3.8–12.3 times more procyanidins (by degree of polymerization DP 1–7) than those in the traditionally Dutch cocoa powder. These flavanol‐rich thinned clusters that are also considered as plant‐based natural products suggested great potential to be functional ingredients in cocoa‐based products—which have been ubiquitously perceived as flavanol‐rich products by consumers—to enhance their overall dietary flavanol content.