Sergey Mergelyan was a Soviet Armenian mathematician, who made major contributions to the Approximation Theory. The modern Complex Approximation Theory is based on Mergelyan's classical work. Corresponding Member Academy of Sciences of USSR (since 1953), member of NAS ASSR (since 1956).

The surname "Mergelov" given at birth was changed for patriotic reasons to the more Armenian-sounding "Mergelyan" by the mathematician himself before his trip to Moscow.

He was a laureate of the USSR State Prize (1952) and the Order of St. Mesrop Mashtots (2008). He was the youngest Doktor nauk in the history of the USSR (at the age of 20),and the youngest corresponding member of the USSR Academy of Sciences (the title was conferred at the age of 24). During his postgraduate studies, the 20-year-old Mergelyan solved one of the fundamental problems of the mathematical theory of functions, which had not been solved for more than 70 years. His theorem on the possibility of uniform polynomial approximation of functions of a complex variable is recognized by the classical Mergelyan theorem, and is included in the course of the theory of functions.

Although he himself was not a computer designer, Mergelyan was a pioneer in Soviet computational mathematics.