Acid sulfate soils face fertility constraints characterized by high acidity and low phosphorus availability. This study aimed to determine the effect of oil palm empty fruit bunch (OPEFB) biochar application on pH, available P, and organic C of acid sulfate soil. A single-factor Completely Randomized Design (CRD) was used with five treatments: 0, 1, 2, 3, and 4 t OPEFB biochar ha?¹. All treatments were incubated in a greenhouse for six weeks. The results indicated that OPEFB biochar application significantly affected pH and available P but did not significantly affect soil organic C. Soil pH increased gradually with the biochar application dosage, from 2.78 (0 t biochar OPEFB ha?¹) to 4.12 (4 t biochar OPEFB ha?¹), with significant differences observed between all treatments. The highest available P (16.47 ppm) was obtained from the 4 tons ha?¹ treatment and differed significantly from the other treatments.  The highest organic C content (8.56%) was descriptively observed at the 3 t biochar OPEFB ha?¹, but the increase was not statistically significant. Therefore, OPEFB biochar at dosage 4 t biochar OPEFB ha?¹ is effective in improving pH and increasing available P in acid sulfate soil during six weeks of incubation.