Algernon Sydney Thelwall (1795 in Newchurch, Isle of Wight – 1863, in St Giles, London) was an evangelical Church of England clergyman and teacher of elocution.
He was named after the 17th-century republican Algernon Sydney showing his father's political leanings.
In 1828 he married Georgiana Anne Tahourdin, and in 1829 became curate of Blackford, near Wedmore, Somerset.
An anti-Catholic, he was active on behalf of the Protestant committee opposing the Maynooth Grant in 1845.
In 1850 Thelwall was appointed Lecturer on Elocution and Public Reading within the theology department of King's College London.