Rad51 is a key enzyme involved in DNA double‐strand break repair by homologous recombination. Here, we show that in response to DNA damage, budding yeast Rad51 is phosphorylated on Ser 192 in a manner that is primarily mediated by the DNA‐damage‐responsive protein kinase Mec1. We show that mutating Rad51 Ser 192 to Ala or Glu confers hypersensitivity to DNA damage and homologous‐recombination defects. Furthermore, biochemical analyses indicate that Ser 192 is required for Rad51 adenosine triphosphate hydrolysis and DNA‐binding activity in vitro , whereas mutation of Ser 192 does not interfere with Rad51 multimer formation. These data suggest a model in which Mec1‐mediated phosphorylation of Rad51 Ser 192 in response to DNA damage controls Rad51 activity and DNA repair by homologous recombination.