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Simple Summary: This study thoroughly investigated viral pathogens associated with bovine respiratory disease complex (BRDC) in Serbian cattle using serum and nasal swab samples. Conducted in 2024 across 65 randomly selected dairy farms in Serbia, excluding ones with vaccinated cattle, this study categorized the farms by their size: small, medium, and large. Serum samples from adult cattle were tested for antibodies against BVDV, BHV-1, BRSV, and BPIV3, while nasal swabs from respiratory-symptomatic animals were PCR-tested for viral genome detection. The results showed seropositivity for all four viruses on all of the farms, with BPIV3 being universally positive. Medium-sized and large farms exhibited higher levels of seropositivity for BRSV and BHV-1 compared to small farms (p < 0.05). Our true seroprevalence estimates were 84.29% for BRSV, 54.08% for BVDV, 90.61% for BHV-1, and 84.59% for BPIV3 at the animal level. A PCR analysis of the nasal swabs detected BRSV (20%), BHV-1 (1.7%), BVDV (8%), and BPIV3 (10.9%), with no Influenza D virus found. This study provides crucial insights into viral pathogen prevalence and circulation in Serbian cattle with BRDC, emphasizing the importance of surveillance and control measures to manage respiratory diseases in cattle populations. Bovine respiratory disease complex, a complex respiratory ailment in cattle, results from a combination of viral and bacterial factors, compounded by environmental stressors such as overcrowding, transportation, and adverse weather conditions. Its impact extends beyond mere health concerns, posing significant economic threats to the cattle industry. This study presents an extensive investigation into viral pathogens associated with BRDC in Serbian cattle, utilizing serum samples and nasal swabs. A cross-sectional study was conducted in 2024 across 65 randomly selected dairy farms in Serbia, excluding farms with vaccinated cattle. The farms were categorized by their livestock count: small (≤50 animals), medium (51–200 animals), and large (>200 animals). Serum samples from adult cattle older than 24 months were tested for antibodies against BVDV, BHV-1, BRSV, and BPIV3. Nasal swab samples from the animals with respiratory signs were tested using PCR for viral genome detection. The results showed seropositivity for all four viruses across all of the farms, with BPIV3 exhibiting universal seropositivity. Medium-sized and large farms demonstrated higher levels of seropositivity for BRSV and BHV-1 compared to small farms (p < 0.05). Our true seroprevalence estimates at the animal level were 84.29% for BRSV, 54.08% for BVDV, 90.61% for BHV-1, and 84.59% for BPIV3. A PCR analysis of the nasal swabs revealed positive detections for BRSV (20%), BHV-1 (1.7%), BVDV (8%), and BPIV3 (10.9%). Influenza D virus was not found in any of the samples. This study provides critical insights into the prevalence and circulation of viral pathogens associated with BRDC in Serbian cattle, emphasizing the importance of surveillance and control measures to mitigate the impact of respiratory diseases in cattle populations. [ABSTRACT FROM AUTHOR]