Objective: This study aimed to compare maternal and cord serum X box binding protein 1 (XBP-1) levels in pregnant women with fetal growth restriction (FGR) with healthy pregnancies. Materials and Methods: This prospective case-control study was conducted between January 1, 2022, and May 31, 2022, at the Gynecology and Obstetrics Clinic of Selçuk University Faculty of Medicine Hospital. Forty-three pregnant women and fetuses with isolated FGR with abdominal circumference (AC) or estimated fetal weight (EFW) below the 10th percentile according to ultrasonographic measurements constituted the study group; 43 healthy pregnant women and fetuses constituted the control group. Serum XBP-1 levels in prenatal maternal and cord blood were measured by ELISA and compared between the two groups. Results: The mean maternal blood XBP-1 level was 1910.22 ± 607.07 ng/L in the FGR group and 1638.89 ± 385.80 ng/L in the control group (p = 0.044). Cord blood XBP-1 levels were 1837.72 ± 942.67 and 1346.14 ± 664.09 ng/L in the study and control groups, respectively (p = 0.006). Maternal XBP-1 levels were found to discriminate FGR with a sensitivity of 80% and specificity of 60% at a value of 1405 ng/L. For cord blood XBP-1, 869.2 ng/L was the best cut-off, with 98% sensitivity and 67% specificity. In ROC analysis, the area under the curve was found to be 0.626 and 0.671, and p values were found to be 0.044 and 0.006 for maternal serum XBP-1 and cord blood XBP-1, respectively. Conclusions: In the study, serum XBP-1 levels in both maternal and fetal umbilical cord blood were found to be higher in patients with FGR. Considering the role of XBP-1 in metabolic pathways and cellular functions, XBP-1 may have an important role in the pathophysiology of FGR.