Toxoplasma gondii causes toxoplasmosis in animals and human and is a model organism for Apicomplexa, a phylum of obligate intracellular parasites including the genus Plasmodium responsible for malaria. Active host cell invasion by these parasites crucially relies on the release of proteins (MICs) from secretory organelles called micronemes onto the parasite surface. MICs engage with receptors on the host cell surface promoting adhesion and internalization. We found that sialic acid, an abundant component of glycoconjugates on all vertebrate cells, is a major determinant for invasion by T. gondii. Two T. gondii MICs, TgMIC1 and TgMIC13, were characterized as the key players of sialic acid-dependent host cell invasion. These two MICs possess domains that adopt a novel fold, termed Microneme Adhesive Repeat (MAR). This domain defines a new protein-family in enteroparasitic coccidians, a subset of apicomplexa including T. gondii. Binding specificities of these proteins might contribute to tissue tropism during infection.