Many key concerns require engagement with science, technology, engineering and mathematics (STEM) knowledge. Consider the complexity and nuance of climate change, energy policy, health and medicine, and data security. Informed voting or decision-making on such issues is no easy task; effective participation in our society requires considerable STEM capabilities. Education of all age groups is essential for a scientifically knowledgeable population involved in making democratic decisions about the future. Lindeman argued for adult education learning processes hand-in-hand with democratic processes, yet, once adults leave formal education, there are woefully few opportunities for the general public to learn or engage with STEM content. Adult education as a field has an important obligation to continue to support adults' STEM learning. This obligation includes helping scientists and STEM educators better connect to adult learners in informal and nonformal settings. This paper explores adult STEM education for democratic participation. We begin with an overview of adult STEM education, followed by STEM knowledge and skills, with a focus on the utility and value of scientific processes and ways of thinking. We then continue with the function of STEM learning in democratic processes followed by frameworks of adult STEM, specifically scientific literacy and place-based education, and conclude with implications and recommendations for adult education practice.