A simple and convenient strategy is described for the in situ synthesis of ultra-small platinum nanoparticles (Pt NPs) at room temperature using poly(4-vinyl phenol) (PVPh) as both the reducing as well as the stabilizing agent in aqueous alkaline solution. This strategy excludes the use of any additional stabilizing agent in addition to use of a reducing agent. Transmission electron microscopic analysis confirms the formation of ultra-small spherical Pt NPs from 1.6 ± 0.2 to 2.2 ± 0.2 nm in diameter with a high degree of monodispersity depending on the ratio of PVPh to platinum salt concentrations used in a single reaction. The as-synthesized ultra-small Pt NPs exhibit extremely high catalytic activity towards the borohydride reduction of p-nitrophenol with very low activation energy (E_a = 24.6 kJ mol⁻¹). Furthermore, the ultra-small PVPh-capped Pt NPs are successfully used as an excellent catalyst for hydrogenation of styrene and nitrobenzene in methanol with very high yield. The PVPh-capped Pt NPs are reusable for up to four cycles of catalysis reaction, although there is a substantial loss of its original activity after the first cycle.