Abstract We study the mass–metallicity relation for 19 members of a spectroscopically confirmed protocluster in the COSMOS field at z = 2.2 (CC2.2), and compare it with that of 24 similarly selected field galaxies at the same redshift. Both samples are H α emitting sources, chosen from the HiZELS narrowband survey, with metallicities derived from the N 2 NII λ 6584 H α line ratio. For the mass-matched samples of protocluster and field galaxies, we find that protocluster galaxies with 10 9.9 M ⊙ ≤ M * ≤ 10 10.9 M ⊙ are metal deficient by 0.10 ± 0.04 dex (2.5 σ significance) compared to their coeval field galaxies. This metal deficiency is absent for low-mass galaxies, M * < 10 9.9 M ⊙ . Moreover, relying on both spectral energy distribution derived and H α (corrected for dust extinction based on M * ) star formation rates (SFRs), we find no strong environmental dependence of the SFR– M * relation; however, we are not able to rule out the existence of small dependence due to inherent uncertainties in both SFR estimators. The existence of 2.5 σ significant metal deficiency for massive protocluster galaxies favors a model in which funneling of the primordial cold gas through filaments dilutes the metal content of protoclusters at high redshifts ( z ≳ 2). At these redshifts, gas reservoirs in filaments are dense enough to cool down rapidly and fall into the potential well of the protocluster to lower the gas-phase metallicity of galaxies. Moreover, part of this metal deficiency could be originated from galaxy interactions that are more prevalent in dense environments.