<p>Red clover (<i>Trifolium pratense</i> L.) is a key grassland legume, and tetraploid cultivars are commonly appreciated for their high productivity and persistence. However, some tetraploid genotypes are self-fertile, which can promote inbreeding, potentially reducing productivity. Despite its importance, the effects of inbreeding on forage and seed production in tetraploid red clover remain poorly characterized. In this study, we generated four successive inbred generations derived from four parental self-fertile genotypes. The impact of inbreeding on forage and seed production was assessed in a two-year field trial and the effects of inbreeding on productivity traits were modelled. Our results demonstrate that stable inbred lines can be developed in tetraploid red clover. Inbreeding reduced forage and seed yield following an exponential decay pattern. Dry matter yield exhibited a half-life value of 3.5 generations, while seed yield and the seed number per flower head declined more steeply with half-life values of 1.4 and 1.5 generations, respectively. Responses were broadly consistent across the four genetic backgrounds, although inbred lines derived from ‘Titus 15’ showed lower reductions in productivity, highlighting potential candidates for hybrid breeding. Given the widespread occurrence of self-fertility in modern-day tetraploid red clover cultivars, breeders should consider the implications for inbreeding, as losses of 1–2% per generation due to self-fertility are possible. Breeders who continue to believe in the added value of tetraploid red clover should prioritize the development of fully self-incompatible cultivars by actively removing self-fertile plants from their breeding populations.</p>