The low photostability of fluorescent proteins is a limiting factor in many applications of fluorescence microscopy. Here we present StayGold, a green fluorescent protein (GFP) derived from the jellyfish Cytaeis uchidae. StayGold is over one order of magnitude more photostable than any currently available fluorescent protein and has a cellular brightness similar to mNeonGreen. We used StayGold to image the dynamics of the endoplasmic reticulum (ER) with high spatiotemporal resolution over several minutes using structured illumination microscopy (SIM) and observed substantially less photobleaching than with a GFP variant optimized for stability in the ER. Using StayGold fusions and SIM, we also imaged the dynamics of mitochondrial fusion and fission and mapped the viral spike proteins in fixed cells infected with severe acute respiratory syndrome coronavirus 2. As StayGold is a dimer, we created a tandem dimer version that allowed us to observe the dynamics of microtubules and the excitatory post-synaptic density in neurons. StayGold will substantially reduce the limitations imposed by photobleaching, especially in live cell or volumetric imaging.