Relentless synonyms such as cruel, assiduous, cold-blooded, and cold-hearted. Relentless is an adjective, and its antonyms are short-lived, irresolute, and intermittent. Additionally, here are 87 synonyms for relentless and words similar to it.
Assiduous, constant, continual, continuing, continuous, determined, diligent, dogged, endless, importunate, incessant, indefatigable, insistent, interminable, intransigent, lasting, never-ending, non-stop, obdurate, obstinate, patient, perpetual, persevering, persistent, pertinacious, purposeful, resolute, sedulous, single-minded, steady, stubborn, sustained, tenacious, tireless, unabated, unabating, unbroken, unceasing, undeviating, unending, unfaltering, unflagging, uninterrupted, unrelenting, unrelieved, unremitting, unstoppable, unswerving, untiring, and unwavering.
Cold-blooded, cold-hearted, cruel, fierce, grim, hard, hard-hearted, harsh, heartless, implacable, inexorable, inflexible, lacking compassion, merciless, obdurate, pitiless, punishing, remorseless, ruthless, severe, stony, stony-hearted, strict, unbending, uncaring, uncharitable, uncompromising, unfeeling, unforgiving, unmerciful, unmoved, unpitying, unsparing, unsympathetic, unyielding, and with a heart of stone.