Signcryption has shown many useful applications, in particular for the environment where the computation and communication resources are constrained, for instance, for applications on lightweight devices. However, we notice that traditional signcryption schemes do not support homomorphic properties, which are very useful in many application scenarios. We also notice that the previous attempt of capturing the homomorphism in signcryption is not provably secure. In this paper, we propose a provably secure additive homomorphic signcryption. Our scheme offers the following two features: (1) Signing and encrypting are carried out in one go, unlike the traditional encryption and signature schemes which are computed separately. (2) We allow the collected signcrypted data items to be aggregated without requiring decryption. The second feature confirms the significance of the first feature in that the traditional signcryption cannot be applied due to lacking of the homomorphic property. Our scheme is the first provably secure signcryption that supports homomorphic property.