StayGold is an exceptionally bright and stable fluorescent protein that is highly resistant to photobleaching. Despite favourable fluorescence properties, use of StayGold as a fluorescent tag is limited because it forms a natural dimer. Here we describe the 1.6 Å structure of StayGold and generate a new derivative (mStayGold) that retains the brightness and photostability of the original protein while being fully monomeric.