This phase 3, 26-week, randomized, double-blind, placebo-controlled, multi-center study will evaluate the safety and efficacy of one dose of Tenapanor in subjects with constipation-predominant irritable bowel syndrome (IBS-C) as defined by the ROME III criteria and who have active disease as determined after a two-week screening period. Subjects who qualify and are randomized into the study will either receive 50mg BID of Tenapanor (1:1) for a 26 week treatment period.