Ostreopsis cf. ovata is a benthic marine microalga recorded along the Italian coasts since the 1990s, being responsible for recurrent bloom events in the Ligurian Sea. Under specific environmental conditions, such as shallow waters, low hydrodynamics and high temperatures, it can form Harmful Algal Blooms (HABs), causing mass mortality of benthic organisms, including sea urchins. However, its effects on early developmental stages remain poorly understood. Within the framework of the Under-Pressure project (PRIN 2022), we investigated the combined effects of thermal stress and O. ovata blooms on the early developmental stages of Paracentrotus lividus. Sea urchin gametes (eggs, sperms) were exposed for up to 72 hours to three O. cf. ovata abundances (4, 40 and 400 cells/ml) under moderate and extreme temperature conditions (21 and 24°C). The effects of HAB exposure on fertilization success (0.2 hours post-fertilization - hpf), embryonic (24 hpf) and larval development (72 hpf) were evaluated. Fertilization occurred in all treatments but was significantly reduced at the highest Ostreopsis abundance (400 cell/ml). Embryonic and larval development were impaired from 40 and 4 cell/ml onward, respectively. A significant interactive effect between temperature and Ostreopsis abundance was observed after 24 hours at 40 and 400 cell/ml, while after 72 hours this interaction was significant across all treatments. In addition, changes in the larvae-assocwasd microbiome induced by O. ovata exposure were investigated through 16S rRNA gene metabarcoding. Overall, the prokaryotic community associated with sea urchin larvae remained stable following combined exposure to Ostreopsis and elevated temperature. However, increasing Ostreopsis abundances resulted in a greater prokaryotic taxonomic richness, altering sea urchin microbiome in the larval stage. In conclusion, our findings demonstrate that high O. cf. ovata abundances negatively affect fertilization success and early developmental stages of P. lividus, while also inducing significant shifts in the associated microbiome.