Jacob or Jakob Nielsen may refer to:

• Jacob Nielsen, Count of Halland (died c. 1309), great grandson of Valdemar II of Denmark
• {{Interlanguage link multi|Jacob Nielsen (businessman)|no|3=Jacob Nielsen}}, Norway (1768-1822)
• Jakob Nielsen (mathematician) (1890–1959), Danish mathematician known for work on automorphisms of surfaces
• Jakob Nielsen (usability consultant) (born 1957), Danish web usability consultant
• Jakob Axel Nielsen (born 1967), Danish lawyer and politician
• Jacob Nielsen (cyclist) (born 1978), Danish cyclist
• Jakob Ahlmann Nielsen (born 1991), Danish footballer

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