Scarlets Head Coach Simon Easterby said that McCusker was a "natural leader" that had earned the respect and support of his colleagues through hard work and commitment.
"Having played alongside Rob and then also coached him as a player, I have seen him develop and mature as a player a great deal over the past six years. He has certainly earned his stripes working extremely hard to get his opportunity with the Scarlets and once he did he hasn't looked back. He is a true Scarlet in terms of his values and commitment to this squad, our region as a whole and its supporters and he will have the full support of all the players in his captaincy.
"He has the respect of his peers and coaching group which is vital and he will lead the team from the front and by example and I know will set the highest of standards along the way."
Easterby added: "We have a young team with some natural leaders in it, and with Rob at the helm it's only going to get better. We thank Matthew Rees who has done a great job over the past two seasons as captain at a very key time in our progression - bringing a level of mental toughness and setting high standards in leading our squad.
"Matthew was intent on seeing the team build strong home form at Parc y Scarlets last season and he continues to add weight and experience within the team as to what we want to achieve in terms of performance. He is totally supportive of Rob being handed this opportunity as captain and is backing him 100% along with all the Scarlets players."
Scarlets will compete in the Pool 5 of next season's Heineken Cup along with reigning champions, Leinster Rugby, Clermont and newcomers Exeter Chiefs.