Enzyme-linked immunosorbent assay (ELISA) methods based on natural enzyme-labeled probes have been applied in the immunoassays, but most have some inevitable limitations (e.g. harsh preparation, purification and storage) and are unsuitable for routine use. Herein we synthesized a new class of irregular-shaped platinum nanoparticles (ISPtNP) with a mean length of 7.0 nm and a narrowing width from 2.0 to 5.0 nm along the longitudinal axes, which were utilized as peroxidase-like mimics for the development of colorimetric immunoassays. Compared with bioactive horseradish peroxidase (HRP), the synthesized ISPtNP exhibited a low Km value (~0.12 mM) and a high Kcat value (~2.27×10(4)s(-1)) for 3,3',5,5'-tetramethylbenzidine (TMB) with strong thermal stability and pH tolerance. The catalytic mechanism of the ISPtNP toward TMB/H2O2 was for the first time discussed and deliberated in this work. Based on a sandwich-type assay format, two types of colorimetric immunoassay protocols were designed and developed for the detection of rabbit IgG (RIgG, as a model) by using the synthesized ISPtNP and conventional HRP as the labeling of detection antibodies, respectively. Similar detection limits (LODs) of 2.5 ng mL(-1) vs. 1.0 ng mL(-1) were obtained toward RIgG with the ISPtNP labeling compared to HRP format. Intra- and inter-assay coefficients of variation were less than 13%. Importantly, the ISPtNP-based assay system could be suitable for use in a mass production of miniaturized lab-on-a-chip devices and open new opportunities for protein diagnostics and biosecurity.