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<i>Pyropia haitanensis</i> is an important laver species in China. Its quality traits are closely related to the content of glutamic acid. Glutamate dehydrogenase (GDH) is a crucial enzyme in the glutamic acid metabolism. In this study, two <i>GDH</i> genes from <i>P. haitanensis</i>, <i>PhGDH</i>1 and <i>PhGDH</i>2, were cloned and successfully expressed in <i>Escherichia coli</i>. The in vitro enzyme activity assay demonstrated that the catalytic activity of PhGDHs is mainly in the direction of ammonium assimilation. The measured <i>K_m</i> values of PhGDH1 for NADH, (NH₄)₂SO₄, and α-oxoglutarate were 0.12, 4.99, and 0.16 mM, respectively, while the corresponding <i>K_m</i> values of PhGDH2 were 0.02, 3.98, and 0.104 mM, respectively. Site-directed mutagenesis results showed that Gly¹⁹³ and Thr³⁶¹ were important catalytic residues for PhGDH2. Moreover, expression levels of both PhGDHs were significantly increased under abiotic stresses. These results suggest that PhGDHs can convert α-oxoglutarate to glutamic acid, and enhance the flavor and stress resistance of <i>P. haitanensis</i>.