The aim of this study was to evaluate the effects of fresh frozen plasma (passive immunotherapy) on clinical and laboratorial findings in puppies treated for neonatal sepsis. A total of thirty-one puppies were included in this study, 16 were diagnosed with sepsis and treated either with plasma + antibiotic (plasma group, n = 7; PG) or antibiotic only (control group, n = 9; CG); whereas 15 were healthy neonates (HG). The groups were evaluated at the first appointment (M0), after 24 (M24), 48 (M48), 72 h (M72) and at the end of treatment (ME). At M0, neonates with sepsis presented a reduction (p < 0.05) in heart rate (HR), reflexes, blood glucose, body temperature, and leukocytes (leukopenia), with an increase (p < 0.05) in lactate and troponin I, compared to HG puppies. During treatment, significant improvement of laboratory parameters was observed in PG compared to CG puppies, with higher IgM level at M24, and higher blood glucose and leukocyte count at M24 and M48 (p < 0.05). At ME, the lactate was lower in PG than CG puppies (p < 0.05). There were no differences in the serum IgG or IgA concentrations among groups. Evaluating clinical and laboratory improvement between time points within the same group, differences (p < 0.05) were observed in the PG puppies, which increased blood glucose, temperature, leukocytes count, and IgM at M24 compared to M0. In the CG, a decrease in IgM concentration was detected at M24 compared to M0 (p < 0.05). Approximately 57% (4/7) of the neonates in the PG presented leukograms without alterations at M24. In the CG, leukograms without alterations were observed only at M72 in 55.5% (5/9) of the animals. The mortality rate was 22% (2/9) in the CG. There was no mortality in the PG. This was the first study using fresh frozen plasma as an adjuvant in the treatment of neonatal sepsis in dogs and evaluation of biomarkers in diagnosis. It was concluded that sepsis led to changes in the physiological parameters of neonatal dogs, demonstrating that the evaluation of leukocytes, troponin I and lactate can be used in the diagnosis and monitoring of sepsis. It was concluded that, besides being an important source of IgM, fresh frozen plasma stimulated healing of puppies with neonatal sepsis, by prompt (within 24 h) improvement in both clinical and laboratory parameters. Plasma therapy has been shown to be beneficial in the treatment of sepsis and is a promising therapeutic strategy for reducing neonatal mortality.