Brain-derived neurotrophic factor (BDNF) is a polypeptide that is secreted from neurons. Although there is mounting evidence that BDNF regulates neuronal development and synaptic plasticity, BDNF secretion has remained unclear due to lack of appropriate methods for the analysis of its dynamics. To visualize BDNF secretion from neurons, here we have developed a cell-based fluorescent indicator for BDNF. We showed that the present cell-based fluorescent indicator, named "Bescell", has high selectivity to BDNF and detects picomolar concentrations of BDNF (detection limit of 60 pM). Bescell has visualized endogenous BDNF secreted from hippocampal neurons. It thus provides a powerful tool for the analysis of BDNF secretion from living neurons.