Mathematical resilience is a student's ability to survive and be positive when facing difficulties and anxiety in learning mathematics as indicated by the ability to work collaboratively with groups, and have language skills to communicate mathematical understanding. Learning mathematics online caused various negative conditions for students; reduced learning interactions, internet disturbances, material that was not optimally explained, even low support for studying at home. This study aims to find out about students' mathematical resilience when taking online mathematics lessons during the COVID-19 Pandemic. A qualitative phenomenological design was used, in which 20 high school students in Medan, participated. Data was collected through an open online questionnaire and interview, due to the implementation of social restrictions in Medan. Thematic analysis was used to interpret the data. The results are summed up: (1) Implementation and use of mathematics learning platforms during the COVID-19 Pandemic; (2) Adaptation of students learning mathematics; (3) Students' belief in learning mathematics; (4) students' communication skills during mathematics learning; (5) The Support for learning mathematics. This study recommends several suggestions to strengthen students' mathematical resilience in order to overcome mathematical difficulties and problem when taking online mathematics learning.