Background: The lumbosacral transitional vertebra (LSTV) is a relatively common congenital anomaly of the vertebrae. LSTV identification is of great importance in presurgical planning and as a cause of lower back pain and early disc degeneration. Overwhelming evidence indicates that whole spine in magnetic resonance imaging (MRI) was capable of counting vertebrae and determining transitional vertebrae. It is also unknown whether the routine lumbosacral MRI protocol is capable of identifying LSTV using the level of iliolumbar ligament (ILL). The purpose of this study was to evaluate the diagnostic value (sensitivity and specificity) of the ILL in identification of LSTV. Methods: Patients underwent whole spine MRI without contrast. Then, accurate numeration of spinal segments starting from 2nd cervical vertebra (C2) was also done as a gold standard for counting vertebrae. In addition, three additional axial T2W cuts from least two vertebral bodies were taken for evaluation of ILL attachment. Among 210 enrolled patients, 178 patients had normal number of vertebra with bilateral ILL attached to the L5, and 32 patients had LSTV (15.2%), of which 29 patients had ILL attached to the L5, and 3 patients did not have ILL; therefore, the ILL specificity and sensitivity were calculated to be 98.3% and 100%, respectively. Conclusion: The ILL revealed considerable specificity and sensitivity for identification of LSTV and can therefore be routinely used in the diagnosis of LSTV in lumbosacral MRI.