Two series of new pyrazoles, namely six pyrazolo[1,5-a][1,3,5]triazine-2,4-dione and four pyrazolo[1,5-c][1,3,5]thiadiazine-2-one derivatives, were synthesized as potential inhibitors of the photosynthetic electron transport chain at the photosystem II level. The compounds were confirmed by 1H NMR, elemental, and IR analyses. Their biological activity was evaluated in vivo upon both the growth of blue-green algae and the photosynthetic oxygen evolution by eukaryotic algae and in vitro as the ability to interfere with light-driven reduction of ferricyanide by isolated spinach chloroplasts. Some compounds exhibited remarkable inhibitory properties, comparable to those of the reference commercial herbicides lenacil, diuron, and hexazinone. Results suggest that the substitution of triazine with thiadiazine ring may act as amplifier for herbicidal activity.