The present investigation analyses the potential of a pedagogical dynamic assessment (DA) approach to foster second language (L2) development through the use of a mobile instant messaging application. Students' zone of actual and proximal development is observed through the use of a grammar and vocabulary level test and the use of the WhatsApp application respectively. Sixty students taking a B1 English course at the language centre of a Spanish university were studied. A mixed methods methodology was used to analyse the differences between two pre-existing groups (control and experimental), each consisting of 30 participants. Both groups received the same tuition and content, and students in the experimental group participated in a daily conversation in the application during a five-month period where negative feedback was provided by the teacher through the use of an inventory of prompts, from most implicit to most explicit. Throughout the research, pedagogical mobile-mediated DA became a central part of the students' learning process, extending learning beyond the in-class time and becoming a constant source of L2 input and feedback. Moreover, results indicated that DA and dialogic mediation helped students reflect on their language performance, gradually requiring less explicit feedback and metalinguistic explanations.