The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late 1880s and is now part of the United States Department of Health and Human Services. Many NIH facilities are located in Bethesda, Maryland, and other nearby suburbs of the Washington metropolitan area, with other primary facilities in the Research Triangle Park in North Carolina and smaller satellite facilities located around the United States. The NIH conducts its own scientific research through the NIH Intramural Research Program (IRP) and provides major biomedical research funding to non-NIH research facilities through its Extramural Research Program.

As of 2013, the IRP had 1,200 principal investigators and more than 4,000 postdoctoral fellows in basic, translational, and clinical research, being the largest biomedical research institution in the world, while, as of 2003, the extramural arm provided 28% of biomedical research funding spent annually in the U.S., or about US$26.4 billion.

The NIH comprises 27 separate institutes and centers of different biomedical disciplines and is responsible for many scientific accomplishments, including the discovery of fluoride to prevent tooth decay, the use of lithium to manage bipolar disorder, and the creation of vaccines against hepatitis, Haemophilus influenzae (HIB), and human papillomavirus (HPV).

In 2019, the NIH was ranked number two in the world, behind Harvard University, for biomedical sciences in the Nature Index, which measured the largest contributors to papers published in a subset of leading journals from 2015 to 2018.