MacDougall was called up to the team this morning after Marcus Di Rollo, the Edinburgh Gunners' centre, failed to recover from a hamstring problem.
Scotland coach Frank Hadden said: "Unfortunately Marcus has not made the progress today that we had hoped but as one door closes, another opens for Ben MacDougall who wins his first cap this weekend.
"I'm very confident that with Ben's big-time rugby experience we will get a top game out of him in Cardiff.
"The reason Ben moved to the Borders last summer was to get the week in, week out rugby that he could not be guaranteed in Edinburgh and the wisdom of his move has been demonstrated throughout the season and underlined today."
MacDougall added: "This is one of the biggest honours in my life. It was a huge decision to switch codes some 16 months ago.
"I've had my good days and bad days but being chosen for Scotland today is definitely one of the best."
MacDougall, who qualifies for Scotland through a paternal grandfather, joined Edinburgh Rugby from Australian rugby league side Melbourne Storm in 2004.