Abstract The MAMMOTH–Grism slitless spectroscopic survey is a Hubble Space Telescope (HST) cycle 28 medium program, which is obtaining 45 orbits of WFC3/IR grism spectroscopy in the density peak regions of three massive galaxy protoclusters at z = 2–3 discovered using the MAMMOTH technique. We introduce this survey by presenting the first measurement of the mass–metallicity relation (MZR) at high redshift in overdense environments via grism spectroscopy. From the completed MAMMOTH–Grism observations in the field of the BOSS1244 protocluster at z = 2.24 ± 0.02, we secure a sample of 36 protocluster member galaxies at z ≈ 2.24, showing strong nebular emission lines ([O III], Hβ, and [O II]) in their G141 spectra. Using the multi-wavelength broadband deep imaging from HST and ground-based telescopes, we measure their stellar masses in the range of [10⁹, 10^10.4] M _⊙, instantaneous star formation rates (SFR) from 10 to 240 M _⊙ yr⁻¹, and global gas-phase metallicities [ 1 3 , 1 ] of solar. Compared with similarly selected field-galaxy samples at the same redshift, our galaxies show, on average, increased SFRs by ∼0.06 dex and ∼0.18 dex at ∼10^10.1 M _⊙ and ∼10^9.8 M _⊙, respectively. Using the stacked spectra of our sample galaxies, we derive the MZR in the BOSS1244 protocluster core as 12 + log ( O / H ) = 0.136 ± 0.018 × log ( M * / M ⊙ ) + 7.082 ± 0.175 , showing a significantly shallower slope than that in the field. This shallow MZR slope is likely caused by the combined effects of efficient recycling of feedback-driven winds and cold-mode gas accretion in protocluster environments. The former effect helps low-mass galaxies residing in overdensities retain their metal production, whereas the latter effect dilutes the metal content of high-mass galaxies, making them more metal-poor than their coeval field counterparts.