Lamb comes into the starting XV following the 24-15 win over Glasgow, with Stephen Myler switching to full-back in place of James Wilson.
The Saints have made a further change on the bench, with Scott Armstrong called into the squad.
Northampton director of rugby Jim Mallinder said: "Having played Castres in both of our last two Heineken Cup campaigns we have an idea of what to expect - some big runners, strong set-piece work and accurate goal-kicking.
"We also know how difficult Castres are to play at home. They have not lost in front of their own fans this season, and even though the game is in Toulouse they will still be very difficult to beat.
"However we are going to France in good spirits after showing our character last Sunday to fight back and secure a bonus point win, and we will be doing everything we can to get the positive result we want."
Castres opened up their campaign with a heavy loss to Ulster - however Northampton will read little into that result.
The Saints suffered a 41-22 away defeat to Castres in last season's competition, while the Top 14 outfit have won 17 of their 26 European games on home soil.
Castres have made wholesale changes to their line-up, though, including a completely new pack.
Mihaita Lazar, Brice Mach, Anton Peikrishvili, Iosefa Tekori, Christophe Samson, Jannie Bornman, Piula Faasalele and Antonie Claassen come in up front - with a further six forwards named on the bench.
In the backs, Romain Martial, Paul Bonnefond, Remi Tales and Rory Kockott are handed starts.
Northampton Saints: 15 Stephen Myler, 14 Ken Pisi, 13 George Pisi, 12 Dom Waldouck, 11 Vasily Artemyev, 10 Ryan Lamb, 9 Lee Dickson, 8 Rhys Oakley, 7 Tom Wood, 6 Phil Dowson, 5 Courtney Lawes, 4 Samu Manoa, 3 Brian Mujati, 2 Dylan Hartley (c), 1 Soane Tonga'uiha.
Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Paul Doran Jones, 19 Mark Sorenson, 20 GJ Van Velze, 21 Martin Roberts, 22 Tom May, 23 Scott Armstrong.
Castres: 15 Romain Teulet, 14 Romain Martial, 13 Paul Bonnefond, 12 Seremaia Bai, 11 Marcel Garvey, 10 Remi Tales, 9 Rory Kockott, 1 Mihaita Lazar, 2 Brice Mach, 3 Anton Peikrishvili, 4 Iosefa Tekori, 5 Christophe Samson, 6 Jannie Bornman, 7 Piula Faasalele, 8 Antonie Claassen.
Replacements: 16 Marc-Antoine Rallier, 17 Yannick Forestier, 18 Michael Coetzee, 19 Matthias Rolland, 20 Daniel Kirkpatrick, 21 Pedrie Wannenburg, 22 Romain Cabannes, 23 Marc Andreu.
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