STW 5, a herbal extract, is effective for the treatment of symptoms in patients with functional dyspepsia (FD). However, its mode of action is still unclear and a modulation of gastric motility is hypothesized. This multicentre, placebo-controlled double-blind study addressed the question of whether STW 5 accelerates gastric emptying in patients with FD and gastroparesis. One-hundred and three patients diagnosed with FD were randomly assigned to a treatment with either STW 5 or a liquid placebo for 28 days. The primary end point of the study was a change of a validated gastrointestinal symptom (GIS) score under treatment. Additionally, patients underwent a 13C octanoic acid breath test for the assessment of the gastric half-emptying time (t1/2). Patients with prolonged t1/2 were diagnosed with gastroparesis and requested to repeat the test at the end of treatment. A change of t1/2 was defined a secondary study end point. t1/2 was prolonged in 48.6% of patients in the STW 5 group and in 43.8% of the placebo group. During treatment, t1/2 increased non-significantly in patients treated with STW 5 (+23 ± 109 min; P = 0.51) and slightly accelerated among patients in the placebo arm (−26 ± 51 min; P = 0.77) (P = 0.49). The improvement of the GIS (P = 0.08) and the proportion of patients with a treatment response (P = 0.03) were more pronounced in the STW 5 group. Our findings suggest that the clinical effects of STW 5 in patients with FD and gastroparesis are not directly mediated by an acceleration of gastric emptying. A clear-cut correlation with symptom improvement is still lacking.