The rapid adoption of the larval zebrafish as a systems neuroscience model has been driven largely by its small size and optical transparency, which enable the use of imaging and optogenetics techniques to record and manipulate activity throughout the brain.Unfortunately, larval fish lack the mature behavioral repertoire of adults and so a number of learning phenomena and social behaviors cannot be investigated in these animals.Here we establish the pedomorphic fish species Danionella translucida as a laboratory model that overcomes this limitation. Adult Danionella possess the size and optical transparency of late-larval zebrafish, which we exploit to image deep within the brains of a behaviorally mature animals using two-photon microscopy. The close phylogenetic relationship between Danionella and zebrafish enabled us to use existing reagents and techniques for transgenesis, and zebrafish-derived enhancer elements drove transgene expression in Danionella with their intended specificities. In a behavioral assay for socially-reinforced place preference, interactions between fish were found to be positively reinforcing and dependent on gender, demonstrating the power of this species for studies of learning and memory as well as social behavior. The establishment of Danionella's behavioral, molecular, and optical tractability provides a unique opportunity for researchers seeking to understand the relationship between circuits and adult-onset behaviors at a whole-brain level.