Pythium porphyrae and Pythium chondricola, collectively known as red rot pathogens, pose a severe threat to nori (Pyropia/Porphyra) cultivation, causing substantial global economic losses. A critical factor underpinning their success as pathogens is their remarkable resilience and adaptability to fluctuating environmental conditions, which enable their persistent survival and infection. In our preliminary work, we annotated the key ectoine biosynthetic gene EctC, which is responsible for the synthesis of ectoine—an important stress-protective metabolite—from the Pythium porphyrae genome; notably, compared with homologs in terrestrial oomycetes, this gene shows a marked expansion Ectoine is a potent stress-protectant molecule that is well characterized in prokaryotes for its role in the osmoprotection and stabilization of macromolecules under various abiotic stresses, including high salinity, drought and oxidative stress. Its presence and potentially functional role in eukaryotic oomycetes, particularly in plant pathogens, represent a paradigm shift, as it was historically considered a prokaryotic-specific metabolite. This study aimed to functionally characterize the role of EctC from P. porphyrae (PpEctC) in oomycete growth, environmental stress tolerance, and pathogenicity. We employed a heterologous functional genomics approach using the established Phytophthora sojae transformation system. We generated a comprehensive set of transgenic strains in strain P6497 to investigate the function of EctC. Using the CRISPR/Cas9-mediated gene replacement strategy coupled with homology-directed repair (HDR), we successfully created a PsEctC knockout mutant (PsΔEctC-RFP), in which the native PsEctC gene was replaced with a red fluorescent protein (RFP) marker. We generated a heterologous complementation strain (PsΔEctC-PpEctC) by replacing PsEctC with the PpEctC gene from P. porphyrae. Additionally, we constructed a strain overexpressing PsEctC (Ps-oeEctC) and strain expressing PpEctC (Ps-hePpEctC) in the wild-type (WT) P. sojae background using a plasmid-based expression system. Successful gene editing, deletion, and altered expression levels of all transgenic strains were validated using PCR, qRT-PCR, and phenotypic screening. Phenotypic characterization under standard conditions revealed that the deletion of PsEctC significantly impaired mycelial growth, as evidenced by significantly smaller colony diameter of the PsΔEctC-RFP mutant compared with that of the wild-type and empty vector control (CK). Intriguingly, heterologous complementation of the PsΔEctC-PpEctC strain with PpEctC fully restored mycelial growth to that of the WT, demonstrating functional equivalence and cross-species compatibility of the P. porphyrae EctC in supporting basic P. sojae vegetative growth. In contrast, neither the knockout nor complementation significantly affected sporangia formation or zoospore production, except for an unexplained reduction in zoospore yield in the PsΔEctC-PpEctC strain. This suggests that EctC is primarily involved in hyphal expansion but not in the specific developmental reproductive stages under non-stress conditions. We then investigated the role of EctC in stress tolerance. Under high salinity stress (35% NaCl), the PsΔEctC-RFP knockout mutant exhibited a dramatic reduction in relative growth, highlighting its increased sensitivity to osmotic stress. The heterologous complementation strain (PsΔEctC-PpEctC) displayed a growth tolerance phenotype, which was statistically indistinguishable from that of the WT and CK, confirming that PpEctC could effectively restore osmotolerance. Strikingly, both the PsEctC overexpression (Ps-oeEctC) and PpEctC heterologous expression (Ps-hePpEctC) demonstrated superior growth under high-salt conditions, significantly outperforming the WT. This indicates that elevated EctC expression, whether from a native or heterologous source, confers a distinct advantage under osmotic stress. A similar trend was observed under alkaline pH stress (pH 9), in which the EctC knockout mutant was severely compromised, whereas the complemented and overexpression mutants maintained robust growth, underscoring the role of ectoine in pH stress mitigation. Given the critical role of reactive oxygen species in plant defense, we assessed the total antioxidant capacity of the transformants. We found that the PsΔEctC-RFP mutant showed a significant decrease in antioxidant capability. Conversely, both the Ps-oeEctC and Ps-hePpEctC mutants exhibited substantial enhancement in their antioxidant capacities, with the latter exhibiting a more potent effect. The complementation strain (PsΔEctC-PpEctC) showed a partial but significant recovery in antioxidant capacity compared to the knockout, although it did not reach that of the WT. This result shows a clear link between the EctC-mediated ectoine synthesis and pathogen’s oxidative stress defense system, which is crucial for countering host-induced oxidative bursts during infection. Pathogenicity assays on etiolated soybean hypocotyls provided compelling evidence for the role of EctC in virulence. The PsΔEctC-RFP knockout strain caused minimal lesions and showed a significantly lower relative in planta biomass than the WT, indicating severely attenuated virulence. Complementation with PpEctC (PsΔEctC-PpEctC) partially restored pathogenicity, leading to a higher biomass than the knockout although not to the WT levels. Most notably, heterologous expression of PpEctC (Ps-hePpEctC) resulted in hypervirulence, with significantly greater pathogen biomass recovered from the infected tissues than from the WT. The overexpression mutant (Ps-oeEctC) also showed enhanced virulence than the WT. These findings strongly suggest that EctC is a critical virulence factor and that its enhanced expression can potentiate pathogen infectivity and colonization ability, likely by bolstering resistance to host-imposed environmental and oxidative stresses. In conclusion, our study provides comprehensive functional evidence that the ectoine synthase gene EctC, particularly the PpEctC variant from P. porphyrae, plays a multifaceted and pivotal role in oomycete biology. It is integral for optimal mycelial growth, essential for tolerance to high salinity and alkaline pH, crucial for enhancing antioxidant capacity, and is a major determinant of pathogenicity. Successful heterologous complementation and the hypervirulent phenotype induced by heterologous expression confirmed functional potency of PpEctC. This study not only elucidates a previously uncharacterized stress adaptation mechanism in a destructive eukaryotic pathogen but also pinpoints EctC and the ectoine biosynthesis pathway as promising and novel targets for developing precise disease control strategies against red rot disease in nori aquaculture. Future work will focus on directly validating these findings in P. porphyrae upon the establishment of a robust transformation system.