Gareth S. Armstrong is a Welsh actor, director, teacher, and writer.

Armstrong began his career by acting in school plays at the Bishop Gore School, Swansea. At the age of 16 he joined the National Youth Theatre; and went on from there to study drama at Hull University. On stage he has played leading roles in most of the UK's regional theatres including Birmingham Rep., Nottingham Playhouse and the Bristol Old Vic. He has specialised in Shakespearean theatre where roles have included Romeo, Richard III, Oberon, Macbeth, Shylock and Prospero. As a member of the Royal Shakespeare Company, he worked in Stratford and London and has performed in the West End in plays by Noël Coward, Tom Stoppard, Agatha Christie and most recently in Yes, Prime Minister (2013). He played at Shakespeare's Globe in 2008, 2010, and 2011.

Armstrong's television credits include: Z-Cars, Doctor Who (in the serial The Masque of Mandragora), Blake's 7, The Professionals, Terry and June, One Foot in the Grave, Casualty and EastEnders and Birds of a Feather.

Armstrong provided the voice for the character of Sandy in the Japanese television series Saiyūki, released in English-speaking countries as Monkey. He has recorded hundreds of audiobooks and has embarked on recording all of Georges Simenon's Maigret novels for Audible. For Black Library he has narrated novels and audio dramas at Games Workshop.

Armstrong's play A Critical Stage, about the theatre critic James Agate, received its premier in June 2023 at London's Tabard Theatre. [citation needed]