Online professional development (OPD) is a popular professional learning option, especially for those with monetary and time constraints such as afterschool educators. This study reports on the design, implementation, and evaluation of an OPD on preparing 18 afterschool educators to teach a content-heavy, transmedia, STEM curriculum on cryptology and cybersecurity to 223 students. The design of our OPD is grounded in andragogy, incorporates OPD best practices, and uses a community of inquiry and online interaction framework. We evaluated the OPD on all four levels (reaction, learning, transfer, and results) of Kirkpatrick's ("Training and Development Journal", 1979) evaluation framework. Educators in general had positive reactions to the OPD, and while reporting they were comfortable with the content, still expressed the desire to review trickier concepts before implementing. When implementing, educators reported they retained content, but still struggled with difficult concepts. Eighteen educators completed the OPD with 17 implementing in their program. One hundred percent of educators who started the curriculum completed it with their students. Students' knowledge assessment scores significant increased (M[subscript]pre = 5.51, SD[subscript]pre = 1.79, M[subscript]post = 7.46, SD[subscript]pos t = 2.39, t = 9.07, df = 143, p < 0.001). Suggested strategies for designers of OPDs with similar curricular and program needs are included.