Most of the baryons in a spiral galaxy are believed to reside in the warm-hot circumgalactic medium (CGM), out to the virial radius of the galaxy. In addition to the Milky Way, the diffuse hot CGM has been detected in X-ray emission only around massive super-L* galaxies, with mass insufficient to close their baryonic budget. By performing two independent analyses using Suzaku and XMM-Newton data, we have now discovered the hot CGM of an L* star-forming spiral NGC 3221 at 3.6σ and >4σ significance respectively. We confirm that the hot CGM is extended beyond 150 kpc with >99.62% confidence. There is a temperature gradient out to 125 kpc, indicating that the CGM is not isothermal. By modelling the surface brightness profile with a β-model, we find that the core radius is >110 kpc, and the mass within 200 kpc is (10±2)×1010 Msolar for a metallicity of 0.3 Zsolar. This is the most massive baryon component of NGC 3221, and can account for the missing galactic baryons of NGC 3221. We will outline a very efficient and systematic method to extract the information and infer the results from any X-ray emission data of the galactic halo, which would enable such exciting discoveries.