The purpose of this study was to analyse the effectiveness of the Flipped Classroom methodology for teaching mathematics, geometry in particular, in an online environment. Specifically, we measured: 1) the academic achievement of students who were taught in this way; and 2) perceptions of satisfaction with this methodology and the virtual sessions attended through digital resources. For this purpose, a pre-experimental and ex-post-facto design was used with a sample of 113 Secondary Education students. With regard to academic performance, positive and significant differences were found between the pre-test and the post-test, with previous underachievement in mathematics, efforts made towards learning and the educational level of the parents being predictors. In respect of the students' perceptions, the results showed a favourable satisfaction with the use of the Flipped Classroom, where the environmental context in which the educational process takes place interferes with their perceptions. However, slightly negative perceptions were found on the use of virtual sessions with digital resources, where the frequency of use of some resources and electronic devices has a significant influence. These results add value on how to further integrate this methodology in the subject of mathematics, where the use of digital resources to learn geometry helps to improve the different competences and skills of the students.