William Trail or Traill FRSE MRIA (23 June 1746 – 3 February 1831) was a Scots-born mathematician, remembered for his mathematical text books.

Through his grandmother Agnes Trail, the younger William was related to the mathematician George Lewis Scott and the two were frequent correspondents.

[3] In 1759, he entered in Marischal College (Aberdeen) and in 1763 he moved to University of Glasgow where he studied under Robert Simson and graduated M.A.

[4] In 1766 he was successful to obtain the chair of mathematics in Marischal College (in competition with John Playfair and Robert Hamilton).

[9] In 1799 he married Lady Frances Charteris (1754-1848) eldest daughter of Francis Wemyss-Charteris and granddaughter of the Duke of Gordon.