Plutonium-240 decays 100 % by alpha transitions to uranium-236 with a half-life of 6561 years. It is a manufactured isotope and can be found in irradiated uranium fuel or spent uranium fuel. Along with 239Pu, 240Pu is a byproduct of nuclear reactor operations and nuclear bomb explosions. The certified massic activity for 240Pu was obtained by 4pα liquid scintillation (LS) counting using three different LS measurement systems (with widely varying operating systems). For the determination two different LS cocktails compositions (with 5 counting sources each) were measured 5 times on each of the three instruments. The combined standard uncertainty (k = 2) on the standardization is 1.1 %. This solution is gravimetrically linked to a 240Pu stock solution obtained from Oak Ridge National Laboratory in February 1980 and was used to prepare the previous NIST 240Pu standard SRM 4338a, which was certified using NIST “0.1 π”α defined-solid-angle counter with scintillation detector and a reference time of 1 May 1996. A remeasurement of SRM 4338a at the time of this work in 2019 agreed with the decay-corrected 1996 certified value to within 0.26 %.