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.