After the outbreak of the Second Boer War in October 1899, Rawson was posted to South Africa where he commanded the Royal Engineers lines of communication in Natal Colony. He was part of the force which relieved Ladysmith in March 1900 and saw action at the Battle of the Tugela Heights and Pieter's Hill. After serving in the Transvaal in May and June, where he was at the Battle of Laing's Nek, he was posted back to Natal, spending some time on the Zululand frontier before receiving the brevet rank of colonel in November 1900. He commanded Royal Engineers in the Pietermaritzburg area during the later part of the war. He was mentioned in dispatches several times and awarded the Queen's South Africa Medal with four clasps and the King's South Africa Medal with both clasps.