His inclusion in the squad is the only change in the match-day 22 compared with the one which beat Fiji 37-25 on Saturday.
The 22-year-old Harley was the obvious candidate among the tour squad to replace John Barclay, the flanker who has gone back home to prepare for his wedding, but his inclusion in the 22 does mean all the uncapped players in the 28 originally selected for the trip could go home having become full Test players.
Prop Ryan Grant and wing Tom Brown won caps against Australia, while Tim Visser and Tom Ryder got theirs against Fiji.
Harley's involvement is far from certain as head coach Andy Robinson is reluctant to use his bench when things are going well.
But in the heat and humidity of Samoa, it is likely he will have little choice about using all his resources against a team who are used to the conditions.
As expected, both Mike Blair and Max Evans, who started against Fiji, are included among the replacements, swapping roles with Chris Cusiter and Sean Lamont who were named in the starting XV earlier this week.
Scotland: S Hogg (Glasgow); S Lamont (Glasgow), N De Luca (Edinburgh), M Scott (Edinburgh), T Visser (Edinburgh); G Laidlaw (Edinburgh), C Cusiter (Glasgow); R Grant (Glasgow), R Ford (C) (Edinburgh), E Murray (Newcastle), A Kellock (Glasgow), R Gray (Sale), A Strokosch (Perpignan), R Rennie (Edinburgh), R Vernon (Sale).
Replacements: S Lawson (London Irish), G Cross (Edinburgh), T Ryder (Glasgow), R Harley (Glasgow), M Blair (Brive), D Weir (Glasgow), M Evans (Castres).