BACKGROUND: Near-infrared spectroscopy is used in the assessment of regional splanchnic oxygen saturation (r s SO 2 ), but solid reference values are scarce. We aimed to establish reference values of r s SO 2 for preterm infants during the first week after birth, both crude and modeled based on predictors. METHODS: We included infants with gestational age (GA) <32 weeks and/or birth weight <1200 g. We excluded infants who developed necrotizing enterocolitis or sepsis or who died. In the first week after birth, we determined a daily 2-h mean of r s SO 2 to assess its associations with sex, GA, postnatal age (PNA), small-for-gestational age (SGA) status, patent ductus arteriosus, hemoglobin, nutrition, and head circumference at birth and translated those into a prediction model. RESULTS: We included 220 infants. On day 1, the mean ± SD r s SO 2 value was 48.2% ± 16.6. The nadir of r s SO 2 was on day 4 (38.7% ± 16.6 smoothed line) to 5 (37.4%±17.3, actual data), after which r s SO 2 increased to 44.2% ± 16.6 on day 7. The final model of the reference values of r s SO 2 included the following coefficients: r s SO 2 = 3.2 − 7.0 × PNA + 0.8 × PNA 2 − 4.0 × SGA + 1.8 × GA. CONCLUSIONS: We established reference values of r s SO 2 for preterm infants during the first week after birth. GA, PNA, and SGA affect these values and need to be taken into account.