A cationic conjugated polyelectrolyte was designed and synthesized based on poly(fluorene‐co‐phenylene) containing 5 mol% benzothiadiazole (BT) as a low energy trap and 15‐crown‐5 as a recognizing group for potassium ions. A potassium ion can form a sandwich‐type 2:1 Lewis acid‐based complex with 15‐crown‐5, to cause the intermolecular aggregation of polymers. This facilitates inter‐chain fluorescence resonance energy transfer (FRET) to a low‐energy BT segment, resulting in fluorescent signal amplification, even at dilute analyte concentrations. Highly sensitive and selective detection of K+ ions was demonstrated in water. The linear response of ratiometric fluorescent signal as a function of [K+] allows K+ quantification in a range of nanomolar concentrations with a detection limit of ≈0.7 × 10−9M.