Background <p>Functional abdominal pain disorders (FAPD) are common in children and there is heterogeneity in the available literature regarding the role of fibre in FAPD and none so-far in functional abdominal pain-not otherwise specified (FAP-NOS).</p> Aim <p>The study aimed to evaluate the efficacy of psyllium fiber in pediatric FAPD including Irritable bowel syndrome (IBS) and FAP-NOS.</p> Methods <p>In this prospective randomized placebo-controlled trial, children (5–18&#xa0;years) diagnosed as FAPD based on Rome IV criteria were randomized to psyllium fiber or placebo (maltodextrin) for 12&#xa0;weeks. Post-treatment improvement of pain and quality of life (QoL) from baseline was compared between the two groups. Pain assessment was done using the pain score table, which assessed duration, frequency, and intensity of pain.</p> Results <p>The mean age of 109 children (FAP-NOS: 85%; psyllium fiber 57, placebo 52) was 11.8 ± 3.9&#xa0;years (58% boys). There was a significant difference in pain improvement in terms of reduction in scores for intensity (5.5 <i>vs.</i> 3.2), frequency (6.0 <i>vs.</i> 3.2), duration (5.3 <i>vs.</i> 3.3), and QoL (4.4 <i>vs.</i> 2.3) between the psyllium and placebo group (<i>P</i> &lt; 0.001 in all). Responders (&gt; 50% reduction in pain) were seen in 86% of the psyllium as compared to 60% in the placebo group (<i>P</i> = 0.001). On multivariate analysis, use of psyllium fiber was the only factor predictive of response (<i>OR:</i> 5.49, 95% <i>CI</i> 2.05–14.72, <i>P</i> &lt; 0.001). After discontinuation of psyllium fiber, 81% had sustained response over a mean follow-up of 6.7 ± 4.0&#xa0;months.</p> Conclusions <p>A 12-week trial of psyllium fiber improves pain in most and gives sustained relief in three-fourths of children with FAP-NOS and IBS.</p> Clinical Trial Registration <p>Clinical Trial Registry of India (&#xa0;CTRI/2023/08/056772; Website <a href="http://www.ctri.nic.in">http://ctri.nic.in</a>).</p> Graphical Abstract <p></p>