Formaldehyde (FA), in the 0.2-0.4 mM range, is produced and maintained endogenously via enzymatic pathways. At these levels, FA can promote cell proliferation as well as mediate memory formation. Once elevated, FA stress is known to induce cognitive impairments, memory loss, and neurodegeneration owing to its potent DNA and protein cross-linking mechanisms. Optical imaging is a powerful noninvasive approach used to study FA in living systems; however, biocompatible chemical probes for FA are currently lacking. Herein, we report the design, synthesis, and biological evaluation of Formaldehyde Probe 1 (FP1), a new fluorescent indicator based on the 2-aza-Cope sigmatropic rearrangement. The remarkable sensitivity, selectivity, and photostability of FP1 has enabled us to visualize FA in live HEK293TN and Neuroscreen-1 cells. We envision that FP1 will find widespread applications in the study of FA associated with normal and pathological processes.