Mathias Bay-Smidt (born 19 March 1996) is a Danish badminton player.
In 2014, he won his first senior international title at the Finnish International tournament in the men's doubles event partnered with Frederik Søgaard Mortensen.
In 2016, he won the Czech International tournament in the mixed doubles event partnered with Alexandra Bøje after fight through the qualification round, with the eight matches played.