Phosphorescent organic light emitting diodes (PHOLEDs) feature high efficiency1,2, brightness, and color tunability suitable for both display and lighting applications3. However, overcoming the short operational lifetime of blue PHOLEDs remains one of the most challenging high-value problems in the field of organic electronics. Their short lifetimes originate from the annihilation of high energy, long-lived blue triplets that leads to molecular dissociation4–7. The Purcell effect, the enhancement of the radiative decay rate in a microcavity, can reduce the triplet density and hence the probability of destructive high-energy triplet-polaron5,6 and triplet-triplet annihilation events4,5,7,8. Here, we introduce the polariton-enhanced Purcell effect in blue PHOLEDs. We find plasmon-exciton-polaritons9 (PEPs) significantly increase the strength of the Purcell effect and achieves an average Purcell factor of 2.4 ± 0.2 over a 50 nm thick emission layer in a blue PHOLED. A 5.3-fold improvement in LT90 (the time for the PHOLED luminance to decay to 90% of its the initial value of a cyan-emitting Ir-complex device is achieved compared to its use in a conventional PHOLED. Shifting the chromaticity coordinates to (0.14, 0.14) and (0.15, 0.20) into the deep blue, the Purcell-enhanced devices achieve 10-14 times improvement over similarly deep blue PHOLEDs, with one structure reaching the longest Ir-complex device lifetime of LT90 = 140 ± 20 h reported to date10-21. The polariton-enhanced Purcell effect and microcavity engineering provide new possibilities for extending deep blue PHOLED lifetimes.