Heavy metal contamination in groundwater has become more prevalent due to the leaching of toxic wastes from various anthropogenic sources. When ingested, it can cause serious ill effects detrimental to human health. Hence, there is a need to monitor the levels of heavy metals in various water sources to ensure they will be fit for human consumption. Standard detection methods such as AAS and ICP-MS are typically used for quantifying the concentration of heavy metals. However, these require expensive equipment, not to mention the need for a trained and highly-skilled technician to operate the equipment. Nanosensors offer a low-cost alternative to these methods. By utilizing the localized surface plasmon resonance (LSPR), properties of noble metal nanoparticles such as AgNPs, colorimetric detection of heavy metals is made possible. Herein, we report the synthesis of humic acid-functionalized silver nanoparticles (HA-AgNPs) by borohydride reduction approach as a colorimetric nanosensor for Ni (II) detection in aqueous solutions. Humic acid acts as a capping agent that stabilizes the AgNPs in the colloidal mixture while providing functional groups for detecting heavy metals. The synthesized HA-AgNPs have an average hydrodynamic diameter of 42.9 nm, a polydispersity index of 0.438, and an LSPR peak of 400.6 nm. The nanosensor can be used for the colorimetric detection of Ni (II) ions within the linear range of 0.15 – 0.40 mM Ni (II) with a limit of detection (LoD) of 2.35 mg L^-1. The HA-AgNPs were shown to be selective in detecting Ni (II) ions; common metals in water such as Ca (II), Mg (II), Al (III), Zn (II), Na (I), and K (I) did not interfere with Ni (II) detection. As such, the HA-AgNPs can be used as a reliable and environment-friendly colorimetric nanosensor for rapid and point-of-need detection of Ni (II) ions in aqueous solutions.