Ollie Lindsay-Hague, who was one of five young players to commit their future to the club this week, starts at full-back while Ben Botica and Karl Dickson partner at half-backs.
In the forwards, Dave Ward starts at hooker, with Charlie Matthews and Sam Twomey completing the front five.
Luke Wallace, who has been shortlisted for this season's "LV= breakthrough player of the year award", captains the side and is joined in the back-row with Tom Guest and Joe Trayfoot.
Among the replacements, Harry Allen will look to make his first appearance for the club, while Ross Chisholm is named after recovering from an ankle injury sustained against Leicester Tigers in September.
Meanwhile, Gordon Ross makes his 100th appearance for London Welsh against Quins.
Ross, 34, who won 25 caps for Scotland, is in his fourth season at the Exiles and has been in tremendous form, having played a pivotal role in helping the club to Championship success last year. Ross joined Welsh from Saracens in 2009 and has so far amassed 525 points for the club, including scoring two tries.
Ross is one of nine changes made by head coach Lyn Jones to the side beaten 39-17 by Stade Francais in Valenciennes last weekend in the Amlin Challenge Cup.
Tom Voyce and Sonny Parker also come into the backs, while in the forwards Paulica Ion starts at tight head and Tom Bristow at loosehead. Elsewhere in the pack, Kirill Kulemin, Ed Jackson and Adam Balding return to the side, while Michael Hills, a first half replacement against Stade, continues in the number seven jersey.
On the bench, there's a welcome return from injury for Martin Purdy and Daniel Browne, while there's also a place for Neath scrum half Luc Jones - son of London Welsh head coach Lyn - who joins the club on loan.
Harlequins: 15 Ollie Lindsay-Hague, 14 Seb Stegmann, 13 Matt Hopper, 12 Tom Casson, 11 Sam Smith, 10 Ben Botica, 9 Karl Dickson, 8 Tom Guest, 7 Luke Wallace (c), 6 Joe Trayfoot, 5 Charlie Matthews, 4 Sam Twomey, 3 Will Collier, 2 Dave Ward, 1 Mark Lambert.
Replacements: 16 Harry Allen, 17 Darryl Marfo, 18 Kyle Sinckler, 19 Jack Clifford, 20 Harry Sloan, 21 Jordan Burns, 22 Louis Grimoldby, 23 Ross Chisholm.
London Welsh: 15 Tom Voyce, 14 Nick Scott, 13 Sonny Parker, 12 Seb Jewell, 11 Phil Mackenzie, 10 Gordon Ross, 9 Alex Davies, 8 Adam Balding, 7 Michael Hills, 6 Ed Jackson, 5 Kirill Kulemin, 4 Adam Brown, 3 Paulica Ion, 2 Dan George, 1 Tom Bristow.
Replacements: 16 Gareth Evans, 17 Billy Moss, 18 James Tideswell, 19 Martin Purdy, 20 Daniel Browne, 21 Luc Jones, 22 Joe Ajuwa, 23 Tom Arscott.
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