A new potassium ion detection assay was developed using a dye-labeled aptamer and conjugated polyelectrolyte (CPE) as a signaling platform via 1-step and 2-step fluorescence resonance energy transfer. Guanine-rich K+-specific aptamers were designed as K+ ion recognition species with 6-carboxyfluorescein (6-FAM) and 6-carboxytetramethylrhodamine (6-TAMRA) at both termini. In the presence of K+ ions, the aptamers undergo a conformational change from an unfolded to folded form by forming a G-quadruplex with K+, bringing two dyes in proximity. FRET-induced 6-TAMRA emission was proportional to [K+] in a range of 22.5 μm–100 mm in water without interference by the presence of excess Na+ ions (100 mm). Upon the addition of CPE, a two-step FRET process from CPE to 6-TAMRA via 6-FAM was enabled, showing an intensified 6-TAMRA signal with K+ ions. The dynamic detection range and limit of detection (LOD) was fine-tuned from ∼millimolar to ∼nanomolar concentrations of K+ by modulating the signal amplification effect of CPE. The LOD was determined to be ≈3.0 nm. This detection assay also showed high selectivity against other metal ions. This sensing scheme can be extended to the detection of a wide range of target materials by simply modifying the recognition aptamer sequence.