This is a Randomized, Double-blind, Placebo-controlled, Multicenter, Parallel-group Study of the Efficacy and Safety of 2 Weight-based Treatment Groups of Plecanatide Versus Placebo in Children and Adolescent Participants 6 to Less Than 18 Years of Age With FC

Inclusion Criteria:• Male or female children and adolescents aged 6 to \<18 years at time of consent. * Diagnosis of functional constipation (FC) based on Rome IV criteria for children/adolescents. * Participant and/or legally authorized representative able to provide informed consent/assent. * Participant/caregiver willing and able to comply with study procedures, including electronic diary (eDiary). * Completion of ≥5 out of 7 daily diary entries during each of the 2 baseline weeks. * Stable diet for at least 14 days prior to screening. * Females of childbearing potential must use highly effective contraception and have negative pregnancy tests. Exclusion Criteria: * Weight \<15 kg at screening/randomization. * History of anorectal malformations, neurological deficits, or anatomical abnormalities affecting bowel function. * Use of prohibited medications within 15 days prior to randomization (e.g., anticholinergics, 5-HT agents, opioids, other laxatives). * Use of any laxatives other than study-provided Dulcolax®. * Pregnant or breastfeeding participants. * Active eating disorder within the past 6 months. * Clinically significant medical conditions (hepatic, renal, gastrointestinal, endocrine, infectious) that may interfere with study. * Known hypersensitivity to plecanatide. * History of drug or alcohol abuse within 12 months. * Participation in another clinical trial within 30 days. * Non-compliance with eDiary requirements (\<5/7 entries per week). * Use of rescue medication more than 2 days per week during baseline. * ≥3 spontaneous bowel movements (SBMs) per week during baseline.