This study investigated a simple, sensitive, and selective method for the colorimetric assay of silver nanoparticles (AgNPs) using Triton X-114-based cloud point extraction (CPE) as a preconcentration step and Tween 20-stabilized gold nanoparticles (Tween 20-AuNPs) as a colorimetric probe. After heating beyond the cloud point temperature of Triton X-114, a solution containing Triton X-114 micelles and AgNPs separated into a surfactant-rich phase (small volume) and a dilute aqueous phase. AgNPs partitioned into a Triton X-114-rich phase through a hydrophobic interaction between Triton X-114 micelles and AgNPs. After phase separation, the concentrated AgNPs oxidized to form Ag⁺ upon adding H₂O₂. The generated Ag⁺ triggered the aggregation of Tween 20-AuNPs in a high-ionic-strength solution because the reduction of Ag⁺ on the AuNP surface enabled Tween 20 (stabilizer) to be removed from the NP surface. The efficiency of Triton X-114-based CPE for the AgNPs was found to be insensitive to their coating type when the AgNPs were modified with a nonionic surfactant and a small ligand. Under optimal extraction and detection conditions, the selectivity of this method for AgNPs was greatly higher than for other nanomaterials. The limit of detection for 10 nm AgNPs was measured to be 1 ng mL⁻¹. This method was successfully applied to the analysis of 10 nm AgNPs in tap water and seawater.