<p>Biogenic gold nanostructures have been obtained, for the first time, by bio-reduction of Au³⁺ ions with an aqueous extract of <i>Opuntia joconostle</i> fruit peel (Oj-AuNPs). This particular methodology is completely green, since room temperature was used, and no infusion preparation was required. The Oj-AuNPs exhibit a broad surface plasmon resonance (SPR) signal, with a maximum at 556&#xa0;nm. SEM and TEM observations show Oj-AuNPs with mean size around 80&#xa0;nm and raspberry-like morphologies, mainly, made of smaller nanoparticles glued by the biomass. When freshly prepared Oj-AuNPs solution is placed in contact with Fe²⁺, the solution changes from brown to a green-grayish color, being the only metal ion changing the Oj-AuNPs solution color among the other twelve metal ions probed, including Fe³⁺. Once the Fe²⁺ ions are detected, the SPR of the Oj-AuNPs becomes broader with a considerable red shift. Furthermore, smaller Au nanostructures, with better defined morphologies than those in the original Oj-AuNPs, are observed by TEM. Therefore, the conceivable mechanism of the naked eye and plasmonic detection of Fe²⁺ by Oj-AuNPs involves the disaggregation of the original larger gold nanostructures. Sensitivity studies of the Oj-AuNPs detection of Fe²⁺ were performed from 200 ppm to 0.1 ppb. The limit of detection (LOD) and limit of quantification (LOQ) for Fe²⁺ are 0.023 and 0.079 ppb, respectively. Moreover, the Oj-AuNPs colorimetric sensor was effectively tested for highly sensitive detection of Fe²⁺ in tap water.</p>