The synthesis of tunable boron nitride (BN) nanotubes is the subject of intense research because of their potential applications in various fields, from smart textile materials to emerging environmental technologies. However, the preparation of these nanomaterials remains extremely challenging. Herein, proof‐of‐concept results on the scalable synthesis of boron nitride nanotubes with tunable dimensions are reported. Combining atomic layer deposition (ALD) of boron nitride on carbon nanofibers template supports and successive annealing steps at high temperature, BN nanotubes with controllable properties are prepared. The successive annealing steps enable the crystallization of BN and the combustion of the initial carbon supports. The self‐limiting nature of the ALD process allows for the scalable coating of large scale carbon fibers matrix. High‐resolution transmission electron microscopy measurements demonstrate the exquisite dimensions control at the nanoscale and the presence of crystalline BN. The proof‐of‐concept results presented allow for independent control of the inner and external diameter dimensions, opening up unique possibilities for the precise engineering of BN nanotubes properties. The nanotubes present good mechanical properties, are easy to handle, and the assessment of their sorption properties shows that they absorb up to 110 times their own weight in oils while repelling water, which opens prospects for water purification.