Aim <p>The present study aims to assess the efficacy of elastic occlusal splints on mobile posterior teeth in patients with stage III or IV periodontitis during Steps 1 and 2 of periodontal therapy.</p> Materials and methods <p>Thirty-eight periodontitis patients, each with at least one mobile maxillary posterior tooth (mobility degree II or III), were randomly assigned to either the Splint group (<i>n</i> = 19), which received maxillary elastic occlusal splints, or the Control group (<i>n</i> = 19), without splints treatment. All patients received Steps 1 and 2 of periodontal therapy. Tooth mobility (TM), masticatory efficiency, probing depth (PD), evaluation of subjective comfort and other parameters were assessed at baseline (T0), 2 ~ 4 weeks (T1) and 6 ~ 8 weeks (T2) after the subgingival scaling. Masticatory efficiency was represented by optical density (OD). The subjective comfort score was recorded using a Visual Analogue Scale (VAS).</p> Results <p>The value of OD in the Splint group was significantly greater than that in the Control group from T0 to T2 (0.19 ± 0.08 versus 0.09 ± 0.09, <i>p</i> &lt; 0.05) and from T1 to T2 (0.11 ± 0.06 versus 0.02 ± 0.06, <i>p</i> &lt; 0.05). Compared with the Control group, patients in the Splint group experienced significantly greater reduction in TM of mobile posterior teeth throughout each treatment period, as well as a more pronounced decrease in PD from T0 to T2. The score of VAS was 8.34 ± 1.10 at T1 and 8.52 ± 0.91 at T2 with elastic occlusal splints.</p> Conclusion <p>The use of elastic occlusal splints as adjuncts during Steps 1 and 2 of periodontal therapy resulted in additional improvement of the masticatory efficiency of periodontitis patients, as well as the reduction of TM and PD of the mobile posterior teeth.</p> Trial registration <p>ChiCTR2400090295.</p>