A new standardization of 90Sr was completed. Strontium-90 is produced by nuclear fission, with a half-life of 28.8 years. It undergoes β− decay into yttrium-90 which in turn undergoes β− decay with a half-life of 2.6684 days. Strontium-90 has applications in medicine; as a radioactive source for superficial radiotherapy of some cancers; and industry as a radioactive source for thickness gauges. It is an isotope of concern in fallout from nuclear weapons, nuclear weapons testing, and nuclear accidents, hence, a good determination is necessary. The certified massic activity for 90Sr in radioactive equilibrium with 90Y was obtained by 4pb liquid scintillation (LS) spectrometry with two commercial LS counters. The LS detection efficiency was calculated using the MICELLE2 code for the CIEMAT/NIST method with composition matched LS cocktails of a 3H standard as the efficiency detection monitor and two different LS cocktail compositions. The combined standard uncertainty (k = 2) on the standardization is 0.30 %. This solution is gravimetrically linked to a 90Sr stock solution (SRM 4234a) used to prepare the previous NIST 90Sr standard SRM 4239. A remeasurement of SRM 4239 at the time of this work in 2020 agreed with the 2008 certified value to within 0.2 %. This determination agrees to the decay corrected SRM 4234a certified value to within 0.002 %.