The　development　of　ultra-high-definition　(UHD)　displays　demands　organic　light-emitting　diodes　(OLEDs)　with　high　color　purity　of　all　three　primary　colors　for　a　wide　color　gamut　and　high　brightness　essential　for　future　AR/VR　applications.　However,　the　vibronic　coupling　in　organic　emitters　typically　results　in　broad　emissions,　with　a　full　width　at　half　maximum　(FWHM)　exceeding　40–50 nm.　Herein,　multicolor　organic　single-crystal　microcavity　light-emitting　diodes　(SC-MC-OLEDs)　are　demonstrated　by　embedding　ultrathin　2D　organic　single　crystals　(2D-OSCs)　between　two　silver　layers　that　serve　as　both　electrodes　and　mirrors.　By　leveraging　the　microcavity　effect,　the　resonant　output　frequencies　of　SC-MC-OLEDs　can　be　continuously　tuned　from　448　to　602 nm　by　adjusting　the　thickness　of　2D-OSCs　(i.e.,　the　microcavity　length),　achieving　high　color　purity　with　a　full　width　at　half　maximum　(FWHM)　of　＜10 nm.　Furthermore,　the　Purcell　effect　in　SC-MC-OLEDs　enhances　the　radiative　rate　and　improves　light-coupling　efficiency,　resulting　in　a　maximum　external　quantum　efficiency　(EQE)　of　up　to　4%　and　minimal　efficiency　roll-off.　Due　to　the　excellent　bipolar　transport　properties　of　OSCs,　the　brightness　of　SC-MC-OLEDs　surpasses　106　cd　m−2,　along　with　a　degree　of　linear　polarization　exceeding　0.9,　unlocking　new　application　opportunities.　©　2025　Wiley-VCH　GmbH.