Tetrapyrrole biosynthesis (TBS) is a dynamically and strictly regulated process. Disruptions in tetrapyrrole metabolism influence many aspects of plant physiology, including photosynthesis, programmed cell death (PCD), and retrograde signaling, thus affecting plant growth and development at multiple levels. However, the genetic and molecular basis of TBS is not fully understood. We report here PCD8, a newly identified thylakoid-localized protein encoded by an essential gene in Arabidopsis. PCD8 knockdown causes a necrotic phenotype due to excessive chloroplast damage. A burst of singlet oxygen that results from overaccumulated tetrapyrrole intermediates upon illumination is suggested to be responsible for cell death in the knockdown mutants. Genetic and biochemical analyses revealed that PCD8 interacts with ClpC1 and a number of TBS enzymes, such as HEMC, CHLD, and PORC of TBS. Taken together, our findings uncover the function of chloroplast-localized PCD8 and provide a new perspective to elucidate molecular mechanism of how TBS is finely regulated in plants.