Lateral size fractionation of niobate nanosheets derived from K4Nb6O17·3H2O was achieved via phase transfer from the aqueous phase to the 2-butanone phase in a water/2-butanone biphasic system, in which tetra-n-dodecylammonium (TDDA+) bromide was used as a phase transfer reagent. Phase transfer of the nanosheets was observed when the TDDA+/[Nb6O17]4- molar ratios were 0.6 and 1.0, and the phase transfer ratios were 41 and 97%, respectively. FT-IR and thermogravimetry results showed that the extracted nanosheets contained TDDA+ ions. These results indicate that adsorption of TDDA+ likely induced an increase in the hydrophobicity of the nanosheet surface, leading to phase transfer. In the AFM image of the original nanosheets in the aqueous phase, their lateral sizes were in the range from several hundreds of nm to several tens of μm, while those of the nanosheets after phase transfer at a molar ratio of 0.6 were in the range from several hundreds of nm up to 2 μm, indicating that nanosheets with smaller lateral sizes were preferentially extracted into the 2-butanone phase. In addition, the phase transfer ratio of the fragmentated nanosheets with a much smaller lateral size distribution compared with the original nanosheets was 79% when the TDDA+/[Nb6O17]4- molar ratio was 0.6, indicating that phase transfer for the nanosheets with smaller lateral sizes proceeded efficiently. Following this extraction cycle, the nanosheets with a TDDA+/[Nb6O17]4- molar ratio of 0.6 remaining in the aqueous phase after extraction were extracted stepwise again through dilution of the aqueous phase with water and the addition of a fresh 2-butanone solution of tetra-n-dodecylammonium bromide to form a new biphasic system. The lateral sizes of the nanosheets increased as the extraction cycles were repeated. Completion of the three extraction cycles allowed formation of the three classes of the nanosheets with different lateral size ranges of 0.68 ± 0.5, 2.8 ± 1.9, and 6.6 ± 3.1 μm.