The purpose of this study was to carry out an analysis of the effectiveness of the Flipped Classroom methodology for teaching mathematics--specifically geometry, in an online environment. Specifically, we measured: (1) the academic achievement of students who were taught based on this method; and (2) the perceptions of satisfaction with this methodology and with the virtual sessions attended through digital resources. In order to gather the data a pre-experimental and ex-post-facto design was used with a sample of 113 secondary education students. Regarding academic performance, positive and significant differences were found between the pre-test and the post-test. In addition, different significant variables were found that affected the academic performance of the students, such as having previously failed the subject of mathematics, the perception of the students about their effort in learning mathematics or the educational level of the parents. In respect of the students' perceptions, the results showed that they were generally happy with the use of the Flipped Classroom methodology, bearing in mind that the environmental context in which the educational process takes place affects the students' perceptions. However, slightly negative perceptions were also found in respect of virtual sessions that made use of digital resources. These findings were significantly affected by the frequency with which resources and electronic devices were deployed. These results would support the integration of this methodology into the teaching of mathematics, where the use of digital resources to study geometry in particular helps to improve the different competences and skills of the students.