A novel palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade of alkene-tethered aryl iodides with α-bromoacrylic acids is reported. This strategy employs α-bromoacrylic acids as a new C1 synthon to assemble diverse heteropolycycles, such as cyclopenta[de]isoquinolinediones and cyclopenta[de]indolo[2,1-a]isoquinolinones, in moderate to excellent yields. This reaction enables the construction of three C–C bonds via sequential fused palladacycle formation, C(vinyl)–Br bond activation and decarboxylation.