Bismuth selenide-based nanomaterials are promising candidates for the use as thermoelectrics, and, in addition to this, are currently studied with respect to their topological insulating behavior. Herein, it is reported on the facile synthesis of composition-tunable colloidal Bi₂Se_x nanocrystals by reacting selenium precursor in dodecanethiol and oleylamine with presynthesized Bi nanoparticles. To the best of our knowledge, this is the first time that such a wide Bi-to-Se ratio-tunability with simultaneous control over morphology has been achieved in the nanocrystalline Bi₂Se_x system with an average crystal size below 40 nm. This method is proved to be successful in producing Bi₂Se₃ compared to employing the coordinating ligand n-trioctylphosphine (TOP). It is observed that Se can be extracted from Bi₂Se₃ using TOP, which can result in an approach to induce a holey structuring into sheet-like Bi₂Se₃. The final products are characterized with respect to their crystal structure, morphology, and composition. Furthermore, their optical characterization reveals a broad absorption in the visible-near-infrared range attributed to a localized surface plasmon resonance.