Security mediated certiflcateless public key cryptography (SMC-PKC) provides certificateless cryptography with instantaneous revocation. Security mediated certificateless signature (SMCS,) is one of the most important security primitives in SMC-PKC. In this paper, we first formalize the definition and security model of security mediated certificateless signatures, and propose a SMCS scheme which is provably secure in the random oracle model. Our scheme satisfies all the desired security properties, has a short public key length, and achieves high efficiency since the signing algorithm needs no pairing operation while the reverse operation only requires one pairing computation.