Sir (Isaac) Lowthian Bell Bart (18 February 1816 - 20 December 1904), Fellow of the Royal Society, was a Victorian Iron Master of Washington, Co. Durham.

In 1854 he built Washington Hall, now called Dame Margaret's Hall. At Washington he established a process for the manufacture of an oxychloride of lead, and built Britain's first plant for aluminium production by the Deville sodium process. With his brothers, he established the major iron works at Port Clarence on the north bank of the river Tees. Throughout his life he studied and published learned works on the chemical basis of iron and steel manufacture.

He was twice Lord-Mayor of Newcastle-upon-Tyne and Member of Parliament for Hartlepool from 1875 to 1880.

In 1895 he was awarded the Albert Medal of the Royal Society of Arts, 'in recognition of the services he has rendered to Arts, Manufactures and Commerce, by his metallurgical researches and the resulting development of the iron and steel industries'.

A founder of the Iron and Steel Institute , he was its president from 1873 to 1875, and in 1874 became the first recipient of the gold medal instituted by Sir Henry Bessemer