Amine-functionalized ultrathin graphitic carbon nitride (g-C3N4) nanosheets were directly obtained by collection of the gaseous thermal polymerization products of urea. The different thermal polymerization mechanisms of urea in the gaseous and solid phases were clarified in this paper. We found that NH3 was crucial for the formation of g-C3N4 nanosheets in the gaseous phase. The obtained g-C3N4 nanosheets were uniform with a large surface area, high reduction potential, and an enhanced charge-carrier separation rate. Thus, these features might promote the catalytic activity and stability of the material for visible-light photocatalytic nitrogen fixation. Moreover, the one-pot synthesis route to amine-functionalized ultrathin g-C3N4 nanosheets might also be applicable as a new chemical vapor deposition method for deposition of g-C3N4 nanosheets.