Benzoxazines with thioamide linkages were successfully prepared. For this purpose, initially, thioamide containing phenolic reagents were synthesized by the Willgerodt-Kindler route using elemental sulfur, aromatic aldehydes and anilines. The obtained phenolic thioamides were then converted to benzoxazines by reacting with primary amines and formaldehyde. Moreover, the thioamide functional polymeric benzoxazine precursor was prepared with difunctional phenolic thioamide and bisaminopropyl end-functional poly(propylene glycol-block-ethylene glycol) by classical Mannich type polycondensation. The resulting precursor was solvent cast on glass plates and flexible films could be obtained after curing. All the synthesized compounds and polymers were characterized by spectral analyses. The curing behavior and thermal stabilities of benzoxazines and related polybenzoxazines were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Interestingly, DSC analyses revealed that the thioamide groups reduce the curing temperatures of benzoxazines by promoting ring opening polymerization, possibly generating thiols during heating.