A　novel　self-solidifying　polymeric　ionic　liquid　(P-[DMT-PS][PHSA])　was　synthesized　via　phenolic　condensation　and　applied　as　a　recyclable　acidic　catalyst　for　biodiesel　production.　The　catalyst　demonstrated　high　acidity　(4.68　mmol/g),　excellent　thermal　stability,　and　effective　catalytic　performance,　achieving　98.19%　oleic　acid　conversion　under　optimized　conditions.　Notably,　P-[DMT-PS][PHSA]　is　soluble　during　the　esterification,　allowing　it　to　act　as　a　homogeneous　catalyst,　and　re-precipitates　upon　methanol　evaporation　for　easy　recovery.　Even　after　10　cycles,　the　catalyst　maintained　97.0%　oleic　acid　conversion,　highlighting　its　stability　and　reusability.　Additionally,　P-[DMT-PS][PHSA]　exhibits　remarkable　substrate　adaptability,　effectively　catalyzing　the　esterification　of　a　range　of　substrates.　This　study　presents　a　simple　and　mild　synthesis　method　for　polymeric　ionic　liquids　and　showcases　P-[DMT-PS][PHSA]　as　a　promising　alternative　to　conventional　catalysts　for　biodiesel　production.　©　2025　Curtin　University　and　John　Wiley　&　Sons　Ltd.