Background: Peganum harmala L. stem ethanolic extract (PHL) has been scarcely evaluated as a ruminant feed component. We evaluated its effects on serum inflammatory and antioxidant biomarkers and duodenal and hepatic molecular responses in Hu sheep. Methods: Twelve male Hu sheep, approximately 120 days old, were randomly assigned to a control group or a PHL treatment group (n = 6 per group). After a 7-day adaptation period, the PHL group received 3 g/head/day of PHL (total flavonoid content: 65.7 mg rutin equivalents (RE)/g; approximately 197.1 mg RE/head/day) for 50 days. Serum markers, duodenal untargeted liquid chromatography–mass spectrometry (LC–MS) metabolomics, liver RNA sequencing, and alternative splicing were analyzed. Results: Serum interleukin-6 and malondialdehyde were lower, whereas interleukin-4 and glutathione peroxidase were higher, in the PHL group than in the control group (p < 0.05). Using variable importance in projection ≥ 1, fold change ≥ 1.2 or ≤0.83, and unadjusted p < 0.05 as screening criteria, 479 LC–MS features met these nominal screening criteria, of which 81 had database annotations. Following Benjamini–Hochberg correction, only two features remained significant (q < 0.05); however, only one was successfully annotated, rendering the pathway analysis exploratory. Liver RNA sequencing identified 279 differentially expressed genes (absolute log₂FC ≥ 1 and q ≤ 0.001) and 104 skipped-exon events (false discovery rate < 0.05). Functional enrichment analysis highlighted annotations related to bile secretion, ATP-binding cassette transport, nutrient metabolism, and glutathione metabolism. Conclusions: PHL supplementation was associated with selective changes in serum inflammatory and antioxidant biomarkers, accompanied by hepatic molecular responses. The duodenal metabolomic findings remain exploratory, and dose–response studies incorporating alkaloid quantification and targeted molecular validation are required before practical application can be considered.