Johnson, who led Scotland in this year's Six Nations campaign, will take up the role of Director of Rugby after the tour.
Glasgow dominate the selection with fourteen players, Edinburgh have nine whilst eight players are 'exiles'.
"First up, it's a strong squad designed to win Test matches against quality opposition.
"We are taking the opportunity to have a look at some players and want to assess what they're like at this level.
"We need to be able to find out whether players who've done the business for their clubs can replicate that form in international footy."
Steven Lawrie, fellow hooker Pat MacArthur and lock Tim Swinson are the new call-ups in the pack.
Alex Dunbar, Peter Horne, Peter Murchie and Tommy Seymour of Warriors, along with Edinburgh full-back Greig Tonks and Saracens wing Duncan Taylor are the fresh faces in the backs.
Scotland leave for South Africa on 31 May and play their first match of a quadrangular tournament, against Samoa, on 8 June.
The Scots face the host nation a week later, with the tournament final and 3rd/4th place match on 22 June.
Alex Dunbar (Glasgow Warriors), Tom Heathcote (Bath Rugby), Peter Horne, Ruaridh Jackson (both Glasgow Warriors), Greig Laidlaw (Edinburgh Rugby), Sean Lamont, Peter Murchie, Henry Pyrgos (all Glasgow Warriors), Matt Scott (Edinburgh Rugby), Tommy Seymour (Glasgow Warriors), Duncan Taylor (Saracens), Greig Tonks and Tim Visser (both Edinburgh Rugby)
John Barclay (Glasgow Warriors), Johnnie Beattie (Montpellier), Kelly Brown (Saracens) (captain), Geoff Cross, David Denton (both Edinburgh Rugby), Alasdair Dickinson (Sale Sharks), Ross Ford, Grant Gilchrist (both Edinburgh Rugby), Ryan Grant (Glasgow Warriors), Jim Hamilton (Gloucester Rugby), Alastair Kellock (Glasgow Warriors), Steven Lawrie (Edinburgh Rugby), Moray Low, Pat MacArthur (both Glasgow Warriors), Euan Murray (Worcester Warriors), Alasdair Strokosch (Perpignan), Tim Swinson and Ryan Wilson (both Glasgow Warriors).