In the present study, we develop a green approach for the preparation of tellurium nanotubes (Te NTs) by a simple fructose-mediated reduction of Te ions in an alkaline aqueous solution. The influence of reaction conditions, including the types of saccharides involved, reaction temperature, and fructose and NaOH concentrations on the size and morphology of Te nanostructures are investigated and the structural evolution with respect to different growth rates are illustrated. It is proposed that a low reduction reaction rate plays a significant role in the controlled growth mechanism of Te NTs. A simple gel-based membrane is also developed for the removal and sensing of mercury ions (Hg²⁺) in aqueous solutions. An agarose gel is used to trap as-prepared Te NTs, leading to the preparation of a nanocomposite film composed of Te NTs-modified agarose gel membrane (Te NTs–AGM) for removing Hg²⁺ in the solution via the strong hybridization and galvanic replacement reaction of Te–Hg. The Te NTs–AGM adsorbent allows effective removal of mercury species spiked in tap, stream, and sea waters with efficiencies greater than 97%. In addition, Te NTs allow for the rapid and simple detection of Hg²⁺ at concentrations as low as 10 nM. We validate the practicality of the use of Te NTs–AGM in this application through analyses of Hg²⁺ in tap water, stream water, and sea water samples. The low-cost, effective, and stable Te NTs–AGM nanocomposite shows great potential for detection and economical removal of Hg²⁺.