He was preparing for a 7th term in the Congo when the First World War broke out and Royaux, despite being too old to be called up, volunteered for service as a Captain in the Reserve. So at 48, he was appointed as commander of the "1ere compagnie" of the "10e régiment de ligne". Present at the Siege of Namur (1914), Royaux distinguished himself in combat and was mentioned in dispatches. After the fall of Namur, he was part of the retreat to Antwerp and took part in the action which halted the German advance at the Battle of the Yser where he was praised by Lieutenant-General Édouard Michel du Faing d'Aigremont, his divisional commander. He was promoted to captain and appointed commander Knight of the Order of Leopold. He was seriously wounded in the left arm at Diksmuide and after recuperating was promoted to Major and was appointed to command the School of Military Interpreters and the Bayeux inspection camp in Normandy. Royaux took part in the offensive to liberate Belgium as commander of the "1ere battalion of 10e regiment de ligne" from 1 September 1918. On 14 October 1918, his unit surprised and captured an entire German battalion and as a result he was awarded the Distinguished Service Order.