Peels from tucumã, a widely consumed fruit in the North of Brazil, were calcined at 800 C and employed as heterogeneous catalyst in biodiesel synthesis. The catalyst characterization by WDXRF showed it was mostly composed of K, P, Ca, and Mg. The catalytic activity was evaluated by transesterification of soy- bean oil with methanol. The transesterification conditions were optimized at 1% w/w of the new catalyst, 4 h reaction time, 80  C, and 15:1 methanol:oil molar ratio, yielding-biodiesel high oil conversion (97.3%). Because of its heterogeneous and non-leachable nature, the catalyst derived from tucum~a peels could be reused 5 times under optimized conditions; a 17.3% decrease of catalytic activity was observed in the fourth reuse. Catalytic activity was also observed in reactions at room temperature and pressure, yielding conversions higher than 80% in 6 h of reaction time with 5% w/w of the catalyst.