A new isoindole alkaloid, 6-hydroxy-2-(4'''-hydroxy-3'''-methoxyphenethyl)-4-(4'-hydroxy-3'-methoxyphenyl)-7-methoxy-1 H -benzo[f]isoindole-1,3(2 H )-dione, named oleraisoindole B was isolated from Portulaca oleracea L., its structure was elucidated using NMR and UHPLC-ESI-Q-TOF/MS spectroscopic methods, and presented anti-inflammatory activity at 5 μM.