Is the US Constitution a primary source?

Yes, the US Constitution is a primary source. In fact, the US Constitution is the main source of law in the US, as it contains the fundamental laws by which the United States is regulated. Moreover, all documents of state, such as the US Constitution, and any federal, state, and local government-issued documents are immediate accounts of information categorized as primary sources.

US constitution
The US Constitution is a primary source. Image source: https://commons.wikimedia.org