Background In the management of lumbar spine diseases, various techniques have been described for minimizing intraoperative blood loss. Soft tissue extravasation and hemolysis have been referred to as hidden blood loss (HBL). By acknowledging HBL and accounting for it in our postoperative care, strategies of fluid infusion and blood transfusion may be altered. Our study aims to estimate HBL in transforaminal lumbar interbody fusion (TLIF) surgeries and to analyze associated factors. Methods This is a retrospective cohort study. Records of patients who underwent TLIF between January 2016 and December 2020 were reviewed. Patients with both minimally invasive (MIS) and open TLIF were included. Patients with infection, tumors, or fractures being the indication for surgery were excluded. Moreover, patients with known blood-related diseases, aged younger than 18 years, patients requiring blood transfusion, or patients with estimated intra-operative blood loss greater than 1.5 L were excluded. HBL was calculated according to the formulae depending on patients’ weight, height, and hematocrit. Statistical analyses were performed to determine associations between HBL and other factors. Results A total of 95 patients were included. The mean estimated blood loss (EBL) was 231 mL, whereas the mean HBL was 265 mL, and the mean total blood loss is 629.7 ml with HBL accounting for 42% of it. Significant associated factors with HBL were the type of surgery, patient’s total blood volume, preoperative hemoglobin and hematocrit, and decrease in hemoglobin and hematocrit. Conclusion Significant HBL may occur after TLIF, which was shown to be more than EBL. Although MIS had less EBL, it was associated with more HBL. Patients’ preoperative hemoglobin and hematocrit, and a decrease in them, have been shown to be associated with HBL. All these factors should be considered for postoperative management of blood loss.