This study investigates teachers' perceptions and practices of designing and teaching transdisciplinary STEAM curriculum using online authentic assessment. Aligning the transdisciplinary STEAM curriculum to authentic assessment is a conceptual framework that guided this study to focus on the three learning domains (cognitive, psychomotor, and affective). The participants are middle and high school teachers (n = 37) in a private school in the United Arab Emirates (UAE). An exploratory mixed method was adopted as a research design. The qualitative data (document analysis) was gathered through analyzing STEAM lesson plans. An online questionnaire was used to collect the quantitative data from teachers. The study's results reveal that teachers' perceptions and practices about the design of STEAM curriculum using authentic assessment were positive with regards to the three learning domains (cognitive, psychomotor, and affective). The highest agreement of teachers' responses was found to be in the affective domain.