Pyrite reactivity with nitrate and nitrite was assessed in long-term batch tests to assess its possible oxidation in anoxic conditions at pH 8.5, at room temperature and atmospheric pressure. This was done in the frame of compatibility studies of nitrate-containing radioactive waste with a pyrite-containing clay host rock for geological disposal. Abiotic pyrite suspensions were prepared under inert atmosphere in 15 mM bicarbonate medium, as this simulates the inorganic carbon and pH conditions in the pore water of Boom Clay, which is a potential host rock for geological disposal in Belgium. Two forms of pyrite powder were used, formed via different genetic pathways and exhibiting a different morphology, namely pyrite obtained by crushing a large crystal cluster and pyrite extracted from Boom Clay by flotation. The reactivity of these two pyrite forms with nitrate and nitrite is reported and compared. Overall, after 2–2.5 years under abiotic conditions and inert atmosphere, no significant reaction between crushed pyrite and nitrate was detected, while a very limited reaction was observed between Boom Clay pyrite and nitrate. Between pyrite and nitrite, which is known to be more reactive than nitrate, a slow reaction took place for both forms of pyrite, with no significantly higher reactivity with the Boom Blay pyrite. The variability between the replicates of Boom Clay pyrite were also larger than for crushed pyrite. Overall, nitrate and nitrite induced a very limited, if any, oxidation reaction of the pyrite powders. These observations are important in assessing the safety of geological disposal of nitrate-containing radioactive waste.