The Ebola virus membrane-associated matrix protein VP40 is thought to be crucial for assembly and budding of virus particles. Here we present the crystal structure of a disk-shaped octameric form of VP40 formed by four antiparallel homodimers of the N-terminal domain. The octamer binds an RNA triribonucleotide containing the sequence 5'-U-G-A-3' through its inner pore surface, and its oligomerization and RNA binding properties are facilitated by two conformational changes when compared to monomeric VP40. The selective RNA interaction stabilizes the ring structure and confers in vitro SDS resistance to octameric VP40. SDS-resistant octameric VP40 is also found in Ebola virus-infected cells, which suggests that VP40 has an additional function in the life cycle of the virus besides promoting virus assembly and budding off the plasma membrane.