Background: Blood pressure readings taken before anesthesia often influence the decision to delay or cancel elective surgeries. However, the implications of these specific blood pressure values, especially how they compare to baseline, on postoperative in-hospital 30-day mortality remain underexplored. This research aimed to examine the effect of discrepancies between the baseline blood pressure evaluated in the ward a day before surgery, and the blood pressure observed just before the administration of anesthesia, on the postoperative mortality risks. Methods: The study encompassed 60,534 adults scheduled for non-cardiac surgeries at a tertiary care center in Seoul, Korea. Baseline blood pressure was calculated as the mean of the blood pressure readings taken within 24 hours prior to surgery. The preanesthetic blood pressure was the blood pressure measured right before the administration of anesthesia. We focused on in-hospital 30-day mortality as the primary outcome. Results: Our research revealed that a lower preanesthetic systolic or mean blood pressure that deviates by 20 mmHg or more from baseline significantly increased the risk of 30-day mortality. This association was particularly pronounced in individuals with a history of hypertension and those aged 65 and above. Higher preanesthetic blood pressure was not significantly associated with an increased risk of 30-day mortality. Conclusion We found that a lower preanesthetic blood pressure compared to baseline significantly increased the 30-day postoperative mortality risk, whereas a higher preanesthetic blood pressure did not. Our study emphasizes the critical importance of accounting for variations in both baseline and preanesthetic blood pressure when assessing surgical risks and outcomes.