Single-layer superconductors are ideal materials for fabricating superconducting nano devices.However,up to date,very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet.Here,using crystal structure search techniques and ab initio calculations,we predict that a single-layer planar carbon sheet with 4-and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature (Tc) up to around 20.8K.More importantly,we propose a synthesis route to obtain such a single-layer T-graphene,that is,a T-graphene potassium intercalation compound (C4 K with P4/mmm symmetry) is firstly synthesized at high pressure (＞ 11.5 GPa) and then quenched to ambient condition;and finally,the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C4K,or peeled off from bulk T-graphite C4,where C4 can be obtained from C4K by evaporating the K atoms.Interestingly,we find that the calculated Tc of C4K is about 30.4K at 0GPa,which sets a new record for layered carbon-based superconductors.The present findings add a new class of carbonbased superconductors.In particular,once the single-layer T-graphene is synthesized,it can pave the way for fabricating superconducting devices together with other 2D materials using the layer-by-layer growth techniques.