Soluble dietary fibres such as oat β-glucan have been suggested to increase gastrointestinal viscosity to improve appetite regulation and reduce subsequent food intake. However, a paucity of research regarding the appetite-regulating effects of β-glucan in semisolid test meals exists. The aim of this study was to examine the appetite-regulating effects of instant rice porridge enriched with oat β-glucan on postprandial satiety and 24-h prospective energy intake. In a crossover, single blind study design, seventeen healthy volunteers consumed instant rice porridge enriched with 1 g oat β-glucan and control rice porridge. Appetite ratings at baseline and during the 210 min following the consumption of test meals were recorded using visual analog scales (VAS). Food diaries were used to record dietary intake prospectively over a 24-h period. No significant differences between treatments were observed in hunger, fullness, satiety, desire to snack and capacity to eat. However, consumption of the β-glucan enriched porridge resulted in a significant reduction (p = 0.026) in energy intake (1715 kJ) in the post-meal 24-h period. Low doses of high-viscosity β-glucan-rich flour in a semisolid meal appeared to decrease prospective energy intake over 24 h, with no postprandial effects on appetite regulation within 210 min.