We present the simple and controllable fabrication of ordered arrays of poly(3-hexylthiophene) (P3HT) solid nanowires and hollow nanotubes by infiltrating the molten polymer into AAO nanopores at temperatures promoting partial (260 C) and complete (280 C) wetting regimes, respectively. We show that such wetting regimes (and thus the formation of nanowires or nanotubes) are associated with a different internal structure in the P3HT melt. At 260 C, the P3HT organizes into a smectic mesophase. Thus, the translational motion of the P3HT molecule through the phase-separated structure would involve an enthalpic penalty, which prevents the molecular diffusion required for achieving the complete wetting regime. Consequently, the P3HT wets the nanopores in partial wetting regime, so that solid nanowires are formed. In contrast, the melt is structurally isotropic at 280 C, which promotes the complete wetting regime, yielding nanotubes. Such a smectic mesophase is also present in P3HT confined into 350 nm in diameter pores. Furthermore, we observe the formation of a new type of nanostructure consisting of twinned nanotubes (two pores formed from one original pore) as a consequence of the appearance of a longitudinal meniscus which divided the hollow interior of the initial nanotube into two independent compartments. Lastly, we use the capillary rise of the P3HT melt along the cylindrical nanopores as a >coarse> nanoscale viscosimetry experiment for the measurement of its viscosity value under confinement. The physical behavior observed for P3HT might be extrapolated to other semiconducting polymers with similar comblike molecular architectures with applications in optoelectronics, thermoelectrics, and photovoltaics (like other poly(alkylthiophenes), polycarbazoles, polyfluorenes, polyphenylenes, etc.). © 2013 American Chemical Society. ERC 2008 Starting Grant “NanoTEC” number 240497. Spanish Ministry Economy and Competitiveness (Project MAT2008-03232 and MAT2011-23455). Peer Reviewed