The Science Mission Directorate (SMD) of the National Aeronautics and Space Administration (NASA) is one of the six directorates that constitute NASA. Its responsibility is to define and direct research into scientific questions of interest, sharing data acquired by NASA missions with the broader scientific community, sponsoring scientific research, and developing satellites and spacecraft in conjunction with other directorates and with partner organizations for scientific missions.

The Science Mission Directorate also sponsors research that both enables, and is enabled by, NASA's exploration activities. The SMD portfolio is contributing to NASA's achievement of the Vision for Space Exploration by striving to:

Nicola Fox is the Associate Administrator for the Science Mission Directorate beginning February 27, 2023. Recent Associate Administrators for the SMD include Edward J. Weiler (1998–2004, 2008–2011), Mary L. Cleave (2004–2005), Alan Stern (2007–2008), John M. Grunsfeld (2012–2016), and Thomas Zurbuchen (2016-2022). Stern resigned 25 March 2008, to be effective 11 April, over disagreements with Administrator Michael D. Griffin.