Gene expression in Plasmodium parasites undergoes significant changes in each developmental stage, but the transcription factors (TFs) regulating these changes have not been identified. We report here a Plasmodium TF (AP2-O) that activates gene expression in ookinetes, the mosquito-invasive form, and has a DNA-binding domain structurally related to that of a plant TF, Apetala2 (AP2). AP2-O mRNA is pre-synthesized by intraerythrocytic female gametocytes and translated later during ookinete development in the mosquito. The Plasmodium TF activates a set of genes, including all genes reported to be required for midgut invasion, by binding to specific six-base sequences on the proximal promoter. These results indicate that AP2 family TFs have important roles in stage-specific gene regulation in Plasmodium parasites.