Synonyms of relentless in English:

relentless


See US English definition of relentless

See UK English definition of relentless

See Spanish definition of implacable

adjective

1‘their relentless pursuit of quality’

SYNONYMS
persistent, continuing, constant, continual, continuous, non-stop, lasting, never-ending, steady, uninterrupted, unabated, unabating, unbroken, interminable, incessant, unstoppable, unceasing, endless, unending, perpetual, unremitting, unrelenting, unrelieved, sustained
unfaltering, unflagging, untiring, unwavering, unswerving, undeviating, persevering, determined, resolute, purposeful, dogged, single-minded, tireless, indefatigable, patient, diligent, assiduous, sedulous, tenacious, pertinacious, insistent, importunate
stubborn, intransigent, obstinate, obdurate
ANTONYMS

short-lived, irresolute, intermittent

2‘a relentless taskmaster’

SYNONYMS
harsh, grim, fierce, cruel, severe, strict, punishing, remorseless, merciless, pitiless, ruthless, unmerciful, unsparing, heartless, hard-hearted, hard, stony-hearted, stony, with a heart of stone, cold-blooded, cold-hearted, unforgiving, unfeeling, uncaring, unsympathetic, uncharitable, lacking compassion, unpitying
inflexible, unbending, uncompromising, obdurate, unyielding, unmoved, inexorable, implacable
ANTONYMS

lenient, merciful