Nanoporous thin films have been extensively studied for their potential applications such as thermoelectrics and thermal management. Different from the popular cylindrical through-film pores, a novel nanoslot pattern design is investigated in this work to achieve better performance in thermal insulation, in-plane thermal anisotropy, and thermoelectrics. In this nanoslot pattern, rectangular pores, or “nanoslots”, are patterned in periodic rows with a narrow neck width between adjacent nanoslots within the same row. Such a narrow neck is the geometric constriction for the phonon transport, resulting in a largely reduced lattice thermal conductivity along the direction perpendicular to the nanoslot rows. In addition, the adjacent rows of nanoslots can also be arranged in an offset manner to further reduce the in-plane thermal conductivity along the same direction, eventually achieving a very high in-plane thermal conductivity anisotropy. Along another line, although the thermal transport is severely suppressed by the nanoslot pattern, the bulk-like electron transport can be largely intact due to the much shorter electron mean free paths compared with the neck width. As a result, the thermoelectric performance can be improved. In this work, frequency-dependent phonon Monte Carlo simulations are utilized to investigate the in-plane thermal conductivity anisotropy of the nanoslot patterns with a maximum room-temperature anisotropy of 37.5 among investigated patterns. Analytical models are also derived to easily predict thermal conductivity and thermoelectric performances. With a neck width reduced to 5 nm, the computed thermoelectric figure of merit can be improved to 0.58 at 1100 K. Potential applications of such nanoslot patterns, such as thermal insulations are heat guides, will also be discussed.