An efficient TE–TM polarization converter built on a silicon-on-insulator nanophotonic platform is demonstrated. The strong cross-polarization coupling effect in air-cladded photonic-wire waveguides is employed to realize the conversion. A peak TE–TM coupling efficiency of 87% ( $-0.6\mathrm{dB}$ insertion loss) is measured experimentally. A polarization conversion efficiency of $>92\%$ with an overall insertion loss of $<-1.6\mathrm{dB}$ is obtained in a wavelength range of $40\mathrm{nm}$ . The proposed device is compact, with a total length of $44\mathrm{\mu m}$ and can be fabricated with one lithography and etching step.