The g-C3N4 nanosheet/TiO2 (CNNS/TiO2) heterojunctions have been synthesized by using polydopamine (PDA) as both linker and inducer enlightened by the bioadhesion and biomineralization mechanisms existing in mussel and shellfish in this study. In the as-prepared CNNS/TiO2 sample, TiO2 nanoparticles about 5 nm in size distribute on the CNNS surface uniformly. Moreover, the heterojunction structure is formed between TiO2 and CNNS. In contrast with CNNS and TiO2, the CNNS/TiO2 heterojunctions display much more superior Rhodamine B (RhB) photocatalytic degradation activity, which can reach 100% within 60 min under irradiation of visible light and preserves excellent stability after four recycles. The heterojunction structure formed by CNNS and TiO2 is conducive for separation of the photogenerated carriers and thus is favorable for improving photocatalytic performance. This method may develop a novel avenue to broadly synthesizing CNNS-based nanocomposites with superior photocatalytic performance.