Piezoelectric energy harvesting is the most effective technique to convert ubiquitous mechanical energy into electricity in comparison to other methods such as triboelectric and electromagnetic based mechanical energy harvesting. This is owing to the high electromechanical coefficient, stability under humidity and environmental changes and suitable range of temperature stability of piezoelectrics. To overcome the limitations of brittleness of piezoceramics and bring the compatibility to the flexible electronics, research on flexible piezoelectric energy harvesters (FPEHs) has become a trend now. Wide varieties of piezoelectric materials are explored in this regard which include conventional ceramic oxides, polymers, biomaterials, etc. Besides, various device designs have enabled performance enhancement and brought compatibility to the different applications. Numerous self-powered sensors based on the FPEHs employed for biomedical sensing, wireless data transmission, environmental remediation, etc. have made this sector of research highly attractive. This review summarizes varieties of piezoelectric materials explored with regard to the FPEHs so far. Then various FPEH device designs are categorically described. Further, possible applications of FPEHs are summarized. Altogether, this review article deals with the summary of materials, device designs and possible applications which gives uniqueness to this article. Performances of different devices are analyzed and discussed. Based on that, future challenges and a roadmap for sustainable FPEH development are outlined.