A novel polyicosahedral nanowire is spontaneously formed in a series of annealing molecular dynamics simulations of liquid Si inside a nanopore of 1.36 nm in diameter. The polyicosahedral Si nanowire is stable even in a vacuum up to about 77% of the melting temperature of bulk Si. Our structural energy calculations reveal that the polyicosahedral nanowire is energetically advantageous over the pentagonal one for a wire whose diameter is less than 6.02 nm, though the latter has been recently proposed as the lowest energy wire. These results suggest the possibility of the formation of a new stable polyicosahedral Si nanowire.