To isolate the genetic locus responsible for saline-alkaline stress tolerance, we developed a high-throughput activation tagging-based T-DNA insertion mutagenesis method using the model rice (Oryza sativa L.) variety Kitaake. One of the activation-tagged insertion lines, activation tagging 7 (AC7), showed increased tolerance to saline-alkaline stress. This phenotype resulted from the overexpression of a gene that encodes a SET DOMAIN GROUP 721 protein with H3K4 methyltransferase activity. Transgenic plants overexpressing OsSDG721 showed saline-alkaline stress-tolerant phenotypes, along with increased leaf angle, advanced heading and ripening dates. By contrast, ossdg721 loss-of-function mutants showed increased sensitivity to saline-alkaline stress characterized by decreased survival rates and reduction in plant height, grain size, grain weight and leaf angle. RNA sequencing (RNAseq) analysis of wild-type Kitaake and ossdg721 mutants indicated that OsSDG721 positively regulates the expression level of HIGH-AFFINITY POTASSIUM (K + ) TRANSPORTER1;5 (OsHKT1;5), which encodes a Na + -selective transporter that maintains K + /Na + homeostasis under salt stress. Furthermore, we showed that OsSDG721 binds to and deposits the H3K4me3 mark in the promoter and coding region of OsHKT1;5, thereby upregulating OsHKT1;5 expression under saline-alkaline stress. Overall, by generating Kitaake activationtagging pools, we established that the H3K4 methyltransferase OsSDG721 enhances salinealkaline stress tolerance in rice.