Knud Aage Nielsen (born 1937) is a retired male badminton player from Denmark.

With a singles game that featured consistency and excellent mobility, Nielsen played at a high international level from the late 1950s through the mid-1960s.

In 1964 he won men's singles at the All England Open Badminton Championships, then considered the unofficial World Badminton Championships, narrowly defeating fellow Dane Henning Borch in the final.

[1] He played Thomas Cup (men's international team competition) singles for Denmark in the '60–'61 and '63–'64 campaigns, scoring Denmark's only singles victory in its controversial '64 Challenge Round loss to Indonesia.

[2] He is the younger brother of Poul-Erik Nielsen three times an All England doubles champion.