Chloroplasts are endosymbiotic organelles of cyanobacterial origin. It seems reasonable to assume that cell division and organelle division still share general principles, as shown for the FtsZ proteins. However, further components involved in this process are largely unknown. Here we describe ARTEMIS, a nuclear-encoded protein of chloroplast inner envelope membranes that is required for organelle division. ARTEMIS consists of three distinct modules: an N-terminal receptor-like region, a centrally positioned glycine-rich stretch containing a nucleoside triphosphate-binding site, and a C-terminal YidC/Oxa1p/Alb3 protein translocase-like domain. Analysis of Arabidopsis En-1 transposon mutants as well as ARTEMIS antisense plants revealed chloroplasts arrested in the late stages of division. Chloroplasts showed clearly separated and distinct multiple thylakoid systems, whereas the final organelle fission remained unaccomplished. Inactivation of a cyanobacterial gene with sequence similarity to the YidC/Oxa1p/Alb3-like domain of ARTEMIS resulted in aberrant cell division, which could be rescued by the Arabidopsis protein. ARTEMIS represents a so-far-unrecognized link between prokaryotic cell fission and chloroplast division.