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Picocyanobacteria represent the main fraction of ocean primary production and, due to the effects of ocean acidification and an increase in seawater temperature, this group is expected to be favored in future scenarios. For this reason, we studied the ecophysiological response of picocyanobacteria to increases in water temperature by testing the allelopathic activity of a subpolar strain of Synechocystis sp., grown at a temperature range of 10-20 degrees C, against coexisting microalgae. We showed that cell-free filtrates of Synechocystis sp. inhibited growth and the maximum quantum yields of PSII (F-v/F-m) of Porphyridium purpureum, Fistulifera sp., and Chlorella vulgaris, and the negative effect of the picocyanobacterium was stronger at the highest temperatures at which Synechocystis sp. was grown. Similarly, in P. purpureum, the effect of filtrates reduced chlorophyll a (Chl a) and carotenoids (Car) content, and the effect depended on the temperature at which the picocyanobacterium was grown. The lipid content was observed to increase in all species, and the allelopathic effect was the strongest at the highest temperature in which Synechocystis sp. was grown. The results of the present study predict a stronger allelopathic effect of picocyanobacteria against competitors with rising temperatures. They also suggest that the potential effect of climate change would benefit this group in future phytoplankton communities.