Three highly efficient blue thermally activated delayed fluorescence (TADF) emitters, 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, have been synthesised based on pyrimidine (PM), 2-methylpyrimidine (MPM) and 2-phenylpyrimidine (PPM) as the acceptor and 10H-spiro[acridan-9,9′-fluorene] (2SPAc) as the donor moiety. With their appropriate molecular design, these emitters successfully achieve small singlet–triplet splitting energies for efficient reverse intersystem crossing, and exhibit high photoluminescence quantum yields. As a result, TADF OLEDs based on 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM emit blue light peaking at 479–489 nm and generate very high external quantum efficiencies of 25.56%, 24.34% and 31.45%, respectively. The 2SPAc-PPM-based TADF OLED is also one of the very few blue TADF emitters with an external quantum efficiency of more than 30%, and it has the best OLED performance among the pyrimidine-based TADF emitters.