Sub-10-nm-sized Pt nanoparticles were synthesized by reducing Pt ions with NaBH4, with the addition of polyethyleneimine (PEI) as a protective reagent. Well-dispersed Pt nanoparticles with a narrow size distribution were successfully produced. When lowering the pH of the PEI solution, cubic and tetrahedral nanoparticles were observed, while at a higher pH, only spherical nanoparticles were produced. In addition, by selecting a suitable concentration of PEI, the protective reagent itself can reduce Pt ions to generate Pt nanoparticles with diameters in the range of 12.9-70.7 nm. This shows the potential of one-pot synthesis of Pt nanoparticles by just adding Pt ions and PEI.