ABSTRACT We use the romulus25 cosmological simulation volume to identify the largest-ever simulated sample of field ultradiffuse galaxies (UDGs). At z = 0, we find that isolated UDGs have average star formation rates (SFRs), colours, and virial masses for their stellar masses and environment. UDGs have moderately elevated H i masses, being 70 per cent (300 per cent) more H i rich than typical isolated dwarf galaxies at luminosities brighter (fainter) than MB = −14. However, UDGs are consistent with the general isolated dwarf galaxy population and make up ∼20 per cent of all field galaxies with 107 < M⋆/M⊙ < 109. The H i masses, effective radii, and overall appearances of our UDGs are consistent with existing observations of field UDGs, but we predict that many isolated UDGs have been missed by current surveys. Despite their isolation at z = 0, the UDGs in our sample are the products of major mergers. Mergers are no more common in UDG than non-UDG progenitors, but mergers that create UDGs tend to happen earlier – almost never occurring after z = 1, produce a temporary boost in spin, and cause star formation to be redistributed to the outskirts of galaxies, resulting in lower central SFRs. The centres of the galaxies fade as their central stellar populations age, but their global SFRs are maintained through bursts of star formation at larger radii, producing steeper negative g −r colour gradients. This formation channel is unique relative to other proposals for UDG formation in isolated galaxies, demonstrating that UDGs can potentially be formed through multiple mechanisms.