BACKGROUND Neck pain (NP) is a common musculoskeletal disorder in primary care that frequently causes discomfort. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to reduce neck pain and associated inflammation and facilitate earlier recovery. Topical diclofenac diethylamine (DDEA) 1.16% gel is clinically proven to be effective and well tolerated in acute and chronic musculoskeletal conditions, but until now no clinical data existed for its use in acute NP. The aim of this study was to assess the efficacy and safety of DDEA 1.16% gel compared with placebo gel in acute NP. METHODS In a randomized, double-blind, placebo-controlled study, patients with acute NP (n = 72) were treated with DDEA 1.16% gel (2 g, 4x/day, for 5 days) or placebo. Efficacy assessments included pain-on-movement (POM), pain-at-rest (PAR), functional neck disability index (NDI) and response to treatment (decrease in POM by 50% after 48 h). Adverse events (AEs) were recorded throughout the study. RESULTS The primary outcome, POM at 48 h, was statistically significantly lower with DDEA gel (19.5 mm) vs. placebo (56.9 mm) (p < 0.0001), representing a clinically relevant decrease from baseline (75% vs. 23%, respectively). All POM scores were significantly lower with DDEA gel vs. placebo from 1 h, as were PAR and NDI scores from first assessment (24 h) onwards (all p < 0.0001). Response to treatment was significantly higher with DDEA gel (94.4%) vs. placebo (8.3%) (p < 0.0001). There were no AEs with DDEA gel. CONCLUSIONS DDEA 1.16% gel, which is available over-the-counter, was effective and well tolerated in the treatment of acute neck pain. The tools used to assess efficacy suggest that it quickly reduced neck pain and improved neck function. However, questions remain regarding the comparability and validity of such tools. Further studies will help ascertain whether DDEA 1.16% gel offers an alternative treatment option in this common, often debilitating condition. TRIAL REGISTRATION ClinicalTrials.gov identifier: NCT01335724.