Rocky exoplanets are particularly abundant around M-type stars. These stars' small radii and low luminosities provide favourable conditions for detecting transiting terrestrial planets and probing their atmospheric properties. We report the discovery and statistical validation of TOI-4616 b, an Earth-sized planet transiting a nearby mid-M dwarf observed by TESS. We confirm the planetary nature of the signal and determine the system parameters by combining TESS photometry with ground-based multi-band transit observations, high-resolution imaging, and optical/near-infrared spectroscopy. The host star lies at a distance of $28.10\pm 0.07$ pc and has $R_\star = 0.1889\pm 0.0096\, \mathrm{R}_\odot$, $M_\star = 0.1881\pm 0.0094\, \mathrm{M}_\odot$, and $T_{\rm eff}=3150\pm 75$ K. TOI-4616 b has a radius of $1.22\, R_{\rm{\oplus }}$ and an orbital period of 1.55 d. The planet receives an incident flux of $\sim 40\, S_{\rm{\oplus }}$, corresponding to an equilibrium temperature of $\sim 525$ K. This places TOI-4616 b in a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. Statistical validation with TRICERATOPS, supported by high-resolution imaging and chromatic transit constraints, yields a false-positive probability of 0.0135 below the recommended threshold of 0.015, confirming TOI-4616 b as a validated planet. Owing to its proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up, TOI-4616 b constitutes a valuable benchmark system for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations.