Binary and multiple star systems are a frequent outcome of the star formation process1;2, and as a result, almost half of all sun-like stars have at least one companion star3. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large scale fragmentation of turbulent gas cores and filaments4;5 or smaller scale fragmen- tation of a massive protostellar disk due to gravitational instability6;7. Observa- tional evidence for turbulent fragmentation on scales of >1000 AU has recently emerged8;9. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems10;11;12;13. The triple protostar system L1448 IRS3B is an ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in an early phase of the star formation process, likely less than 150,000 years in age14, and all protostars in the system are separated by