The contents of total polyphenols, flavonoids, phenolic acids, anthocyanins, and carotenoids were determined using spectrophotometric and chromatographic methods, alongside antioxidant activity: 2,2-diphenyl-1-picrylhydrazyl (DPPH), Ferric Reducing Antioxidant Power (FRAP), Cupric Reducing Antioxidant Capacity (CUPRAC), and hydroxyl radical scavenging assays). Additionally, the levels of antinutritional compounds (tannins, phytates, oxalates, alkaloids, and saponins) were assessed in the flowers of five pumpkin species: giant pumpkin, summer squash, butternut squash, fig-leaf gourd, and cushaw squash (Cucurbita maxima, C. pepo, C. moschata, C. ficifolia, and C. argyrosperma). The results revealed significant interspecific variation in both bioactive and antinutritional compounds. Giant pumpkin flowers exhibited the highest content of polyphenols and phenolic acids, fig-leaf gourd flowers were the richest in carotenoids, whereas butternut squash flowers had the highest anthocyanin levels. The strongest antioxidant activity was observed in giant pumpkin flowers, which can be attributed to their high phenolic and flavonoid content. Despite the presence of moderate amounts of antinutritional compounds, pumpkin flowers can be considered a valuable edible raw material with nutraceutical potential.