Translocations are a well-established tool to combat biodiversity loss yet still face a great deal of scrutiny. Two arguments against translocations are that genetic augmentation will swamp out local genetic variants and increasing population density will increase threatening processes, particularly disease. There is currently little evidence to support either of these arguments, but they are continually used to push back against translocation proposals. Arguably, doing nothing has far greater consequences as isolated populations continue to decrease and succumb to the genetic pressures of small population size. Here we use the release of 33 Tasmanian devils to an isolated, diseased population of less than 20 animals to investigate genetic diversity changes and prevalence of devil facial tumour disease between 2016 and 2021. Released animals successfully bred with incumbent individuals, tripled population size, improved genome-wide diversity and introduced 26 new functional genetic variants to the population and with no common genetic variants lost. We identified 12 genotypes that were associated with improved immunogenetic fitness, all of which were either introduced, increased, or maintained during the reinforcement. Disease prevalence remained consistent throughout the project. Combined, our results show that over eight years of monitoring, approximately three devil generations, there was no evidence for swamping out local genetic variants or increasing the prevalence of DFTD. Instead, we improved demographic structure, genetic diversity, and adaptive potential of a critically small population. Here we show carefully planned translocations can be used for populations facing strong selective pressures and infectious diseases.