Abstract Background Context Sitting spinal alignment is increasingly recognized as a factor influencing strategy for deformity correction. Considering that most individuals sit for longer hours in a ‘slumped’ rather than erect posture, greater understanding of the natural sitting posture is warranted. Purpose To investigate the differences in sagittal spinal alignment between two common sitting postures: a natural, patient-preferred posture and an erect, investigator-controlled posture that is commonly used in alignment studies. Study Design/Setting A randomized, prospective study of 28 young, healthy patients seen in a tertiary hospital over a 6-month period. Patient Sample Twenty-eight patients (24 males, 4 females), with a mean age of 24 years (range 19-38), were recruited for this study. All patients with first-episode of lower back pain of less than 3 months duration were included. The exclusion criteria consisted of previous spinal surgery, radicular symptoms, red flag symptoms, previous spinal trauma, obvious spinal deformity on forward bending test, significant personal or family history of malignancy and current pregnancy. Outcome Measures Radiographic measurements included sagittal vertical axis (SVA), lumbar lordosis (LL), thoracolumbar angle (TL), thoracic kyphosis (TK) and cervical lordosis (CL). Standard spino-pelvic parameters (PI, PT and SS) and sagittal apex and end vertebrae were also measured. Methods Basic patient demographics (age, gender, ethnicity) were recorded. Lateral sitting whole spine radiographs were obtained using a slot scanner in the imposed erect and the natural sitting posture. Statistical analyses of the radiographical parameters were performed comparing the 2 sitting postures using chi-squared tests for categorical variables and paired t-tests for continuous variables. Results There was forward SVA shift between the 2 sitting postures by a mean of 2.9cm (p<0.001). There was a significant increase in CL by a mean of 11.62° (p<0.001), and TL kyphosis by a mean of 11.48° (p<0.001), as well as a loss of LL by a mean of 21.26° (p<0.001). The mean PT increased by 17.68° (p<0.001). The entire thoracic and lumbar spine has the tendency to form a single C-shaped curve with the apex moving to L1 (p=0.002) vertebra in the majority of patients. Conclusion In a natural sitting posture, the lumbar spine becomes kyphotic and contributes to a single C-shaped sagittal profile comprising the thoracic and the lumbar spine. This is associated with an increase in CL and PT, as well as a constant SVA. These findings lend insight into the body's natural way of energy conservation utilizing the posterior ligamentous tension band while achieving sitting spinal sagittal balance. It also provides information on one of the possible causes of PJK/ PJF.