The ongoing language reform in the Estonian education system is establishing Estonian as the only medium of instruction in early childhood education and care (ECEC) settings. This change to bilingual preschool education, which has existed since the collapse of the Soviet Union in 1991, affects the 30% Russian-speaking minority population and is a critical and timely issue for immersion teachers’ practice and professional development (PD). Despite the extensive literature highlighting teachers’ PD in general and in the context of language immersion classrooms, there is a significant lack of literature covering the topic of the PD needs of language immersion ECEC teachers in Estonia which is the focus of this study. Adopting a transformative-emancipatory paradigm, a mixed methods approach was utilised in a four-stage research design with 49 participants in total which included:a quantitative teachers’ survey (Stage 1); a semi-structured interview prior to the intervention (Stage 2); an intervention in the form of a PD programme and classroom observations (Stage 3); and a post-intervention semi-structured interview (Stage 4). The findings were processed quantitatively and qualitatively; the qualitative data were analysed with the help of the NVivo software using reflexive thematic analysis. The research results indicate that the current PD provision requires development concerning its accessibility, especially to teachers with different proficiency levels in the Estonian language, while PD content needs to focus on play-based pedagogy and developing teachers’ Estonian language proficiency. The outcomes also show that teachers need PD events that not only enhance their professionalism but also allow them to improve Estonian language skills and to receive emotional support. The research findings can contribute to better comprehension of ECEC teachers’ PD needs in Estonia with regard to the transition to the Estonian-medium preschool education for 3-7-year-old children.