William Forbes-Sempill, 17th Lord Sempill

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 1873 he was promoted to Colonel Commandant and presented with a sword inscribed "Presented to Colonel Commandant Sir William Forbes, Bt of Craigievar, on his promotion, by the 1st Ad Battn, Aberdeenshire RV, as a mark of their friendship and esteem 1873".

In 1884, he inherited the title of Lord Sempill from his relation Maria Jane Sempill, 16th Lady Sempill, and adopted the surname of Sempill.

He married three times, the first time in 1858 to Caroline, the only daughter of Sir Charles Forbes, 3rd Baronet; this ended in divorce in 1861, after the birth of one daughter, Katherine. Katherine married the naturalist George Muirhead FRSE in 1907.

The second marriage was in 1862 to Frances Emily Abercromby, the youngest daughter of Sir Robert Abercromby, 5th Baronet, in 1862. The youngest of the four, Arthur, was in the Royal Navy, and survived the Battle of Jutland. The elder daughter, Evelyn, married Duncan Vernon Pirie MP, whilst the younger, Gertrude, never married.

The third marriage was in 1890, to Mary Sherbrooke.