Aim To assess the effectiveness of transanal irrigation (TAI) compared with posterior tibial nerve stimulation (PTNS) in severe and chronic low anterior resection syndrome (LARS). Method A two‐group parallel, open‐label randomized controlled trial carried out in a single university hospital. The study population included patients with a LARS scale score of more than 29 points who had undergone rectal surgery more than 1 year previously. These were randomly allocated, with a central randomization system, following a 1:1 sequence to TAI or PTNS. The main study outcome was to achieve a reduction of at least one LARS grade in at least 50% of the patients, for each intervention. Results A total of 27 patients (TAI = 13, PTNS = 14) were randomized. Both groups were similar with regard to confounding factors. Four patients were excluded because of intercurrent disease or early dropout, leaving 23 (TAI, n = 10; PTNS, n = 13) for analysis. Eight out of 10 and 4 out of 13 patients were downgraded with TAI and PTNS, respectively. The median LARS score decreased from 35 [interquartile range (IQR) 32–39] to 12 (IQR 12–26) (P = 0.021) for the TAI group and from 35 (IQR 34–37) to 30 (IQR 25–33) (P = 0.045) for the PTNS group. The Vaizey score fell from 15 (IQR 11–18) to 6 (IQR 4–7) (P = 0.037) and from 14 (IQR 13–17) to 9 (IQR 7–10) (P = 0.007) with TAI and PTNS, respectively, with 80% and 38% of patients, respectively, showing decreases of more than 50%. Improvement in quality of life was observed in both groups. Conclusion Both treatments improved the LARS score in this study but this was only significant in the TAI group.