Serum lactate dehydrogenase (LDH) is a recognized prognostic biomarker in human lymphomas, yet its clinical significance in canine lymphoma remains insufficiently characterized. This study aimed to quantify serum LDH levels in healthy dogs and dogs with high-grade multicentric lymphoma (ML) (predominantly B-cell) and to investigate correlations between LDH levels and established clinical and laboratory prognostic indicators. Twenty-seven dogs were prospectively enrolled: healthy controls (G1, n = 7) and dogs with high-grade ML (G2, n = 20). Immunophenotyping was performed by immunohistochemistry (CD3/CD79a). LDH concentrations were measured at diagnosis (T0) and after six weeks of CHOP-based induction chemotherapy (T1). Statistical analyses included Kruskal–Wallis, Wilcoxon signed-rank, Pearson’s correlation, and mixed-effects models. Dogs with high-grade ML exhibited significantly elevated LDH levels compared to controls (median 545.5 U/L, range: 288.2–2816 U/L vs. 143 U/L, range: 66–272; p < 0.001). Dogs with thrombocytopenia had higher baseline LDH (median 746 U/L, range: 612–921; p = 0.006) and greater reductions following chemotherapy (median −1011.7 U/L, range: −159 to −2064; p = 0.004). LDH levels declined significantly after treatment (overall median reduction 50.7%; post-chemotherapy range: 60.4–752 U/L; n = 15; p = 0.013), with normalization achieved in 77.8% of dogs with complete response versus 16.7% with partial or progressive disease (p = 0.02). We confirmed that serum LDH is significantly elevated in dogs with high-grade ML and declines following effective chemotherapy, supporting its utility as a dynamic biomarker of tumor burden and treatment response. Thrombocytopenic dogs may represent a biologically distinct subset warranting further investigation.