This study compared two versions of the same elementary science professional development (PD) and curriculum program that were offered face-to-face and completely online, while keeping all content consistent between the two. Using quantitative analyses of pre- and posttests of content knowledge (CK), we evaluated the extent to which the online version of the PD compared to the face-to-face PD for increasing the earth science and art CK of upper elementary teachers required to teach an earth science and art integrated curriculum. Additionally, we explored how the impact of PD modality (online vs. face-to-face) on teachers' CK learning outcomes differed for novice and experienced teachers. Findings revealed significantly higher CK learning gains for teachers who participated in online PD compared with face-to-face PD, but with a small effect size. Subgroup analysis revealed that compared with experienced teachers, the novice teachers had significantly higher CK gains from participating in the online PD compared to the face-to-face version with a large effect size. We also discuss the implications for the design of large-scale online teacher PD.