Sean will be making his Scotland Test debut and Donnie will win his third cap at openside flanker. Sean scored his first senior Scotland try in last weekend's defeat to Queensland.
"I didn't have an inkling that I'd get this chance, but it was a very pleasant surprise to be picked," said Sean.
"It's been a great season for me - coming to Glasgow was the best move I made."
Andy Henderson is at inside centre and Gordon Buloch is at hooker with Jon Petrie and Dan Parks on the bench.
"Sean has been in very good form for Glasgow, he has won a lot of awards there this season, and had a very good game against high quality opposition with Queensland," said Scotland coach Matt Williams.
"He has earned his place in the side. He has been in our squads from the Italy game on in the Six Nations. We have tried to edge him into the side."
Scotland suffered another injury blow as centre Tom Philip has been forced to withdraw from the touring squad due to a serious knee injury sustained during training.
Scotland team doctor James Robson said: "Following consultation with an orthopaedic specialist and a scan on Tom's knee, the decision has been taken for him to return to Scotland. He has sadly ruptured his anterior cruciate ligament and will now require surgery followed by approximately nine months of intensive rehabilitation before returning to the game."
Scotland team to play Samoa, Friday 4 June, Westpac Stadium, Wellington, 7.35pm
15 Chris Paterson (Edinburgh Rugby)
14 Sean Lamont (Glasgow Warriors)
13 Ben Hinshelwood (Worcester)
12 Andy Henderson (Glasgow Warriors)
11 Simon Webster (Edinburgh Rugby)
10 Gordon Ross (Leeds Tykes)
9 Chris Cusiter (The Borders)
1 Tom Smith (Northampton Saints)
2 Gordon Bulloch (Glasgow Warriors)
3 Bruce Douglas (The Borders)
4 Stuart Grimes (Newcastle Falcons)
5 Scott Murray (Edinburgh Rugby) CAPTAIN
6 Jason White (Sale Sharks)
8 Allister Hogg (Edinburgh Rugby)
7 Donnie Macfadyen (Glasgow Warriors)
Substitutes
16 Steve Scott (The Borders)
17 Craig Smith (Edinburgh Rugby)
18 Iain Fullarton (Sale Sharks)
19 Jon Petrie (Glasgow Warriors)
20 Mike Blair (Edinburgh Rugby)
21 Dan Parks (Glasgow Warriors)
22 Hugo Southwell (Edinburgh Rugby)
Travelling with the party: Gareth Morton (The Borders) and Scott Gray (Bath Rugby)