It follows yesterday's news that the 20-year-old has signed his first full contract, keeping him at Ulster until at least June 2015.
Henderson's inclusion at blindside flanker will mean Stephen Ferris switches to the number 8 position. The rest of the team is unchanged from last Friday's derby victory over Connacht.
The only change on the replacements bench will see Ruan Pienaar, who returned from Rugby Championship duty with South Africa earlier in the week, taking over from Michael Heaney.
After missing most of last season with an Achilles tendon injury, full-back Jared Payne will also make his European bow.
Looking ahead to the game, Henderson said: "I can't wait. I've been watching games at Ravenhill since I was a kid and I'm really looking forward to pulling on the white jersey and experiencing the atmosphere as a player.
"Castres are a very physical team but they also throw the ball wide and have an expansive kicking game. That should make for a tough but entertaining game. We'll have to work really hard to nullify their threats."
Castres currently occupy sixth position in the Top 14 having beaten Clermont Auvergne last weekend, and with Ulster undefeated in the RaboDirect PRO12, it promises to be a tantalising clash. Former Ravenhill crowd favourite Pedrie Wannenburg is likely to line up for the French side.
Ulster v Castres is a 'ticket only' match, with only 500 tickets remaining - all of which are positioned behind the posts at either end.
Ulster Rugby are currently experiencing difficulties with their phone system so all fans wishing to purchase tickets are advised to book online at www.ulsterrugby.com.
ULSTER (v Castres Olympique): Jared Payne; Tommy Bowe, Darren Cave, Paddy Wallace, Andrew Trimble; Paddy Jackson, Paul Marshall; Tom Court, Rory Best, John Afoa, Johann Muller (capt), Dan Tuohy, Iain Henderson, Chris Henry, Stephen Ferris.
Replacements: Rob Herring, Callum Black, Adam Macklin, Lewis Stevenson, Mike McComish, Ruan Pienaar, Luke Marshall, Craig Gilroy.