William Forbes-Sempill, 17th Lord Sempill (20 May 1836 – 21 July 1905), born William Forbes, was a Scottish peer, the 17th Lord Sempill and 8th Baronet of Craigievar. He was the son of Sir John Forbes, 7th Baronet, and succeeded his father to the baronetcy on his death in February 1846, at the age of seven. He was educated at Eton and then joined the Coldstream Guards in 1854; he served in the Crimean War, rising to the rank of captain. In 1884, he inherited the title of Lord Sempill from his relation Maria Jane Sempill, 16th Lady Sempill, and adopted the surname of Sempill.