Trump had long ago telegraphed his interest in Masters as the Republican nominee after pressuring Arizona Attorney General Mark Brnovich for nearly a year to file criminal charges
In a race where many Republican voters have told pollsters they remained undecided, Trump's backing could take on added significance.
Trump's endorsement prominently cited the border and the 2020 election as important factors in the Aug. 2 primary where the winner will face incumbent Sen. Mark Kelly, D-Ariz.
"Blake is Strong on Border Security, in particular, the disastrous Southern Border where people are pouring into the U.S. by the millions, and destroying our Country," Trump said in a write statement
"Just two years ago, with the help of the Wall, we had the Strongest Border in the history of our Country, and now we have the weakest — Blake will turn that around quickly.
Trump has had more success in endorsements for the U.S. House of Representatives this year. Arizona's Senate race could improve Trump's endorsement record.
That's because the limited polling from independent sources suggests many GOP voters haven't settled on anyone in sizable numbers.
Trump's endorsement remains highly coveted for Republicans running for office, but he hasn't shown an ability to reshape races.