This article is dedicated to examining a bilateral project established between South Korea and Türkiye with the overarching objective of elevating awareness and endowing preschool teachers with the capacity to embed education for sustainability within their classroom practices seamlessly. For this purpose, a professional development (PD) program was developed regarding the three pillars of sustainability and the 7R themes. Fourteen preschool teachers attended the PD program. Three teachers participated in this case study to reveal teachers' transformations regarding sustainability and early childhood education for sustainability. The data were collected through pre- and post-follow-up interviews and classroom observations. Thematic content analyses were conducted. According to the results, the PD program enhanced and deepened teachers' sustainability knowledge. In addition, teachers could transfer the knowledge from the PD program on education for sustainability into their practice not entirely but acceptably, and they have a chance to reconsider their lifestyle habits regarding sustainability in many areas.