The Edinburgh club travel to St Mary's to face Saints on July 25, before taking on Championship side Plymouth at Home Park on July 29.
Hearts will be based in England for a week-long training camp as they prepare for both the SPL and Europa League next season.
"These are two good and strong games and I think they will be perfect for our preparation," manager Csaba Laszlo told Hearts News.
"We are looking forward to working very hard for a week in England getting ready for the new season.
"I am told that our supporters will also enjoy these games, so hopefully there will be many Hearts fans at Southampton and Plymouth."