Costelow starts at fly-half for Wales Under-18s in the International Series fixture against England who include his club colleagues Martin and Whitcombe among the replacements in Stellenberg.
The Welsh were narrowly beaten by hosts South Africa Schools on Friday, while England were also on the receiving end as they lost 41-21 against France despite a try from Tigers second-rower Martin.
Kick-off is at 1.15pm BST and there is live coverage on S4C.
Wales U18: 15 Ioan Lloyd (Clifton/Bristol/Exiles), 14 Harri Doel (Coleg Sir Gar/Scarlets), 13 Osian Knott (Coleg Sir Gar/Scarlets), 12 Aneurin Owen (Ysgol Gwynllyw/Dragons), 11 Louis Rees-Zammit (Hartpury/Gloucester/Exiles), 10 Sam Costelow (Oakham/Leicester Tigers/Exiles), 9 Ellis Bevan (Bryanston/Exiles); 1 Callum Williams (Maes y Gwendraeth/Scarlets), 2 Cameron Lewis (Coleg Sir Gar/Ospreys), 3 Ben Warren (Coleg y Cymoedd/Cardiff Blues), 4 Teddy Williams (Glantaf/Cardiff Blues), 5 Jac Price (Coleg Sir Gar/Scarlets), 6 Oliver White (Llandovery College), 7 Jac Morgan (c) (Coleg Sir Gar/Scarlets), 8 Carwyn Tuipulotu (Sedburgh/Scarlets)
Replacements: 16 Llew Phillips-Taylor (Coleg Sir Gar/Scarlets), 17 Luke Yendle (Newport High/Dragons), 18 Garyn Phillips (Colege Gwent/Dragons), 19 Archie Griffin (Marlborough/Bath/Exiles), 20 Rob Brookson (Coleg Gwent/Dragons), 21 Ioan Davies (Coleg y Cymoedd/Cardiff Blues), 22 Morgan French (Bromsgrove/Worcester/Exiles), 23 Dan Babos (Dragons Academy/Dragons), 24 Evan Lloyd (Coleg Gwent/Dragons), 25 Ewan Rosser (Coleg Gwent/Dragons), 26 Morgan Richards (Newport High/Dragons), 27 Joe Roberts (Coleg Sir Gar/Scarlets)
England U18s: 15 Will Haydon-Wood (Newcastle Falcons/Sedbergh School), 14 Jack Bates (Bristol Bears/SGS Filton), 13 Tom Roebuck (Sale Sharks/Wirral Grammar), 12 Phil Cokanasiga (London Irish/St Pauls College), 11 Josh Gillespie (Northampton Saints/Millfield School), 10 George Barton (Gloucester Rugby/Dean Close School), 9 Raffi Quirke (Sale Sharks/St Ambrose College); 1 Rowan Mullis (Gloucester Rugby/Hartpury College), 2 Sam Riley (Harlequins/St George's Weybridge), 3 Jack Hughes (Northampton Saints/Uppingham School), 4 George Hammond (Harlequins/Whitgift School), 5 Chunya Munga (London Irish/Leighton Park School), 6 Archie Smeaton (Yorkshire Carnegie/Hymers College), 7 Fin Rossiter (London Irish (Wellington College)(c), 8. Jack Clement (Gloucester Rugby/Cheltenham College)
Replacements: 16 Alfie Barbeary (Wasps/Bloxham School), 17 James Whitcombe (Leicester Tigers/Woodhouse Grove), 18 Harvey Beaton (Saracens/Sutton Valence), 19 George Martin (Leicester Tigers/Brooksby Melton College), 20 Harry Dugmore (Harlequins/Whitgift School), 21 Nahum Merigan (Bath Rugby/Beechen Cliff School), 22 Callum Pascoe (Northampton Saints/Millfield School), 23 Tom Curtis (Sale Sharks/Sedbergh School), 24 Josh Hallett (Saracens/New Hall School), 25 Louis Lynagh (Harlequins/Hampton School), 26 Lennox Anyanwu (Harlequins/Christ's Hospital)