Study Design: Reproducibility study. Objectives: To report the agreement and reliability for commonly used sagittal plane measurements. Summary of Background Data: Spinopelvic parameters and sagittal vertical axis (SVA) are commonly used parameters for preoperative planning and postoperative evaluation of patients with adult spinal deformity (ASD). Previous reproducibility studies have focused on describing the reliability using intraclass correlation coefficients (ICCs), thus quantifying the methods' ability to distinguish between individuals. To our knowledge, no previous study in patients with ASD has reported the measurement error in terms of limits of agreement. The current study aimed to report the agreement and reliability for measurements of pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and SVA in ASD patients. Methods: In a consecutive, one-center cohort of 64 patients referred for ASD evaluation, a blinded test-retest study was performed. Reliability was assessed using ICCs, whereas 95% limits of agreement (LOAs) were used to quantify agreement. Results: We found “excellent” (ICC > 0.9) results in all analyses of reliability except for interrater PI, which was classified as “good” (ICC = 0.89). However, considerable interrater measurement error was observed for parameters depending on the angulation of the sacral end plate (95% LOA of ±11° and ±14° for SS and PI, respectively) compared with ±5° for PT and ±7 mm for SVA, which depends on the location of the sacral end plate. Intrarater agreement was only slightly better. Conclusion: These are to our knowledge the first estimates of measurement error for sagittal spinopelvic parameters in ASD patients. Despite near excellent ICCs, we found considerable measurement error for parameters depending on the angulation rather than the location of the sacral end plate. Level of Evidence: Level II.