Introduction: Physiotherapists commonly use the manual inclinometer and Flexicurve for the clinical measurement of thoracic spinal posture. The aim of this study is to examine the concurrent validity of the Flexicurve and manual inclinometer in relation to the radiographic Cobb angle for the measurement of thoracic kyphosis. Methods: Eleven subjects (seven males, four females) underwent a sagittal plane spinal radiograph. Immediately following the radiograph, a physiotherapist measured thoracic kyphosis using the Flexicurve and manual inclinometer before the subjects moved from position. Cobb angles were subsequently measured from the radiographs by an independent examiner. Results: A strong correlation was demonstrated between both the Cobb angle and the Flexicurve angle (r = 0.96) and the Cobb angle and the manual inclinometer angle (r = 0.86). On observation of the Bland–Altman plots, the inclinometer showed good agreement with the Cobb angle (mean difference 4.8 ° ± 8.9 °). However, the Flexicurve angle was systematically smaller than the Cobb angle (mean difference 20.3 ° ± 6.1 °), which reduces its validity. Conclusion: The manual inclinometer is recommended as a valid instrument for measuring thoracic kyphosis, with good agreement with the gold standard. While the Flexicurve is highly correlated to the gold standard, they have poor agreement. Therefore, physiotherapists should take caution when interpreting its results.