The purpose of this study was to conceptualize a pedagogical content knowledge framework for informal science educators (ISE-PCK). An initial, hypothetical framework was created grounded in the literature in informal science education (ISE) as analogous to PCK models in formal science education. An exploratory sequential mixed-methods research design was used to revise, refine, and validate the initial framework. Data were collected through interviews and Delphi method surveys with practicing ISE educators and education researchers. The refined and validated ISE-PCK framework comprised five, correlated constituent components: knowledge of ISE goals, knowledge of ISE programs, knowledge of context(s) of audience(s), knowledge of engagement strategies, and knowledge of evaluation. Of these five components, knowledge of engagement strategies was considered the most important and knowledge of evaluation was considered the least important, as ranked by the Delphi study participants. Articulation of a framework for ISE-PCK is useful to professionalize the important work that ISE educators do as well as to provide guidance for professional development programs. This framework for ISE-PCK will serve as a conceptual foundation for future research focusing on informal science educators' knowledge in relation to their performance and their professional growth from novice to expert.