Higher yield potential and greater yield stability are common targets for crop breeding programs, including those in rice. Despite these efforts, biotic and abiotic stresses continue to impact rice production. Rice blast disease, caused by Magnaporthe oryzae, is the most devastating disease affecting rice worldwide. In the field, resistant varieties are unstable and can become susceptible to disease within a few years of release due to the adaptive potential of the blast fungus, specifically in the effector (avirulence or AVR) gene pool. Here, we analyzed genetic variation of the effector gene AVR-Pik in 58 rice blast isolates from Thailand and examined the interaction between AVR-Pik and the cognate rice resistance gene Pik. Our results reveal that Thai rice blast isolates are very diverse. We observe four AVR-Pik variants in the population, including three previously identified variants, AVR-PikA, AVR-PikD, and AVR-PikE, and one novel variant which we named AVR-PikF. Interestingly, 28 of the isolates contained two copies of AVR-Pik, always in the combination of AVR-PikD and AVR-PikF. Blast isolates expressing only AVR-PikF show high virulence to rice cultivars encoding allelic Pik resistance genes, and the AVR-PikF protein does not interact with the integrated HMA of the Pik resistance protein in vitro, suggesting a mechanism for immune evasion.