This report describes a colorimetric assay for trivalent metal cations (M(3+)) using gold nanoparticles (AuNPs)-modified with oppositely charged thiols that can form intermolecular zwitterionic surfaces. Zwitterionic AuNPs (Zw-AuNPs) are stable in high-salt solutions and well-dispersed in a wide range of pH values. M(3+) including Fe(3+), Al(3+), and Cr(3+) can effectively trigger the aggregation of Zw-AuNPs by interfering with their surface potential, and aggregated AuNPs can be regenerated and recycled by removing M(3+). In our approach, the output signal can be observed by the naked eye within a micromolar (μM) concentration range. Uniquely, our assay is capable of discriminating Fe(3+) from Fe(2+), which is challenging using traditional approaches. More importantly, Zw-AuNPs can be stored stably at room temperature for a long period (3 months) with constant detection performance. Both the cost-effectiveness and the long shelf life make Zw-AuNPs ideal for detecting M(3+) in resource-poor and remote areas.