The global pandemic due to COVID-19 has changed the way of teaching, and Online Learning came to the forefront. The aim of this study is to investigate primary school teachers' perceptions on Online Learning during the COVID-19 pandemic. A case study was conducted with 14 teachers from 5 Primary Schools in Greece. A thematic analysis of the semi-structured interviews showed six themes: Motivation, Support, Strategies, Benefits, Challenges and the Future of Online Education. First of all, teachers showed their initial stress, progressively their high motivation and, at the end, their tiredness. In addition, teachers perceived as important the support from different groups of people, including students' parents, colleagues, the school principal and family members. They also used a variety of instructional strategies for the implementation of Online Learning, such as the use of asynchronous online platforms, the use of educational videos and internet-based learning materials. Moreover, teachers perceived that they have benefited from this experience by digitising new teaching material and by maintaining the learning process. However, they have faced a considerable number of challenges, such as lack of resources, technical problems, lack of training and difficulty in the participation of all the students. Finally, teachers were uncertain about the future of Online Learning without denying its possible use as a complementary tool. To conclude, implications for practice and for future research are discussed.