Diatoms are single-celled photosynthetic eukaryotes responsible for CO2 fixation and primary production in aquatic ecosystems. The cosmopolitan marine diatom Coscinodiscus granii can form seasonal blooms in coastal areas and interact with various microorganisms, including the parasitic oomycete Lagenisma coscinodisci. This unicellular eukaryote is mainly present in the northern hemisphere as an obligate parasite of the genus Coscinodiscus. Understanding the interplay of abiotic factors such as temperature and biotic factors like parasitism on algal physiology is crucial as it dictates plankton community composition and is especially relevant in climate change. This study investigates the impact of two temperatures, 13°C and 25°C, on Coscinodiscus granii under laboratory conditions. A decreased infection rate of the parasite was observed at the elevated temperature. Comparative metabolomic analysis using UHPLC-HRMS revealed that temperature and parasitism significantly affect the algal metabolome. Abundances of metabolites related to sulfur metabolism, including cysteinoleic acid and dimethylsulfoniopropionate, as well as molecules linked to fatty acid metabolism, e.g., carnitine, acetylcarnitine, and eicosapentaenoic acid, significantly increase in cells grown at a higher temperature, suggesting the enhanced rate of metabolism of host cells as temperature rises. Our study reveals how temperature-induced metabolic changes can influence host-parasite dynamics in a changing climate.