Aims: To assess the pharmacokinetic profile of ivermectin in Biłgorajska geese (Anser anser domesticus) after single I/V or oral administration, in order to compare these routes of administration and assess oral bioavailability. Methods: Ten healthy male geese were used in a single-dose, two-phase study with a 3-month washout period between phases. In the first phase, all geese were given 0.2 mg/kg I/V ivermectin, while in the second phase they were treated orally with the same dosage. Blood samples were collected at selected time points up to 480 hours after each administration. Samples were purified using protein precipitation and drug concentration was quantified using HPLC. The analytical method was validated on blank goose plasma and was characterised by an optimal linearity and a limit of quantification of 0.025 μg/mL. The pharmacokinetic analysis was carried out using a non-compartmental approach. Results: The drug was quantifiable up to 240 hours after I/V administration, while after oral treatment it was quantifiable up to 144 hours in most of the geese. The elimination half-life of ivermectin was approximately 3.8 (95% CI = 1.98–7.92; p = 0.027) times higher after I/V administration compared to oral administration. Moreover, the area under the curve from zero to the last detectable timepoint was 6.4 (95% CI = 4.65–8.74; p < 0.001) hours greater after I/V than oral administration. This difference led to a bioavailability of 20.38 (SD 5.92) %. Conclusions: Following oral administration in geese, ivermectin has a bioavailability of approximately 20%. Further research on the action of ivermectin in the gastrointestinal tract is required along with assessment of tissue residues to allow calculation of withdrawal time to ensure consumer safety. Abbreviations: AUC: Area under the concentration-time curve; AUClast: Area under the curve from zero to the last detectable timepoint; AUMC: Area under the first mo