Necrotizing enterocolitis (NEC) remains a frequent catastrophic disease in preterm infants, and fecal filtrate transfer (FFT) has emerged as a promising prophylactic therapy. This study explored the role of virome viability for the protective effect of FFT. Using ultraviolet (UV) irradiation, we established a viral inactivation protocol and administered FFT, UV-inactivated FFT (iFFT) or sterile saline orally to preterm piglets at risk for experimental NEC. The gut pathology and barrier properties were assessed, while the microbiome was explored by 16S rRNA amplicon and metavirome sequencing. Like in prior studies, FFT reduced NEC severity and intestinal inflammation, while these effects were absent in the iFFT group. Unexpectedly, piglets receiving FFT exhibited mild side effects in the form of early-onset diarrhea. The FFT also converged the gut colonization by increased viral heterogeneity and a reduced abundance of pathobionts like and . In contrast, the gut microbiome of iFFT recipients diverged from both FFT and the controls. These findings highlight the clear distinction between the ability of active and inactivate viromes to modulate gut microbiota and decrease pathology. The efficacy of FFT may be driven by active bacteriophages, and loss of virome activity could have consequences for the treatment efficacy.