This paper describes the nonlinear optical and structural properties of the platinum (Pt) colloidal nanoparticle solution in water. A nanosecond fibre laser (1064 nm) with a fluence of 61.1 J/cm$^2$ was used to produce platinum nanoparticles (Pt NPs) in deionised water. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements were used to study the structural and morphological characteristics of the produced Pt NPs. The linear optical properties were examined by UV–visible spectrophotometry. The size distribution of NPs was determined using dynamic light scattering (DLS). Using the DLS data, the Mie scattering theory was employed to predict the plasmonic behaviour of Pt NPs. The Z-scan technique was used for measuring the nonlinear refractive index and absorption coefficients. It was found that the colloidal solution of Pt NPs in water exhibits negative nonlinear refraction and reverse saturable absorption (RSA) properties. The optical limiting threshold intensity of the Pt NPs was also determined. The optical limiting threshold power was determined to be about 30±2 mW. The good optical limiting performance of the Pt NPs can lead to their use in protecting human eyes and delicate optical sensors from unintended exposure to high-power laser radiations.