Holger Bech Nielsen (born 25 August 1941) is a Danish theoretical physicist and professor emeritus at the Niels Bohr Institute, at the University of Copenhagen, where he started studying physics in 1961.

Nielsen has made original contributions to theoretical particle physics, specifically in the field of string theory.

Several physics concepts are named after him, e.g. Nielsen–Olesen vortex and the Nielsen-Ninomiya no-go theorem for representing chiral fermions on the lattice.

In a series of papers uploaded to arXiv.org in 2009, Nielsen and fellow physicist Masao Ninomiya proposed a radical theory to explain the seemingly improbable series of failures preventing the Large Hadron Collider (LHC) from becoming operational.

The collider was intended to be used to find evidence of the hypothetical Higgs boson particle.