Synonyms for Indefatigable:

adj.

all (adjective)
unflagging, unwearying, tireless.
continuous (adjective)
persistent, seamless, monotonous, constant, regular, flat, steady, unbroken, unvarying, unceasing, continuous, monolithic, consistent, homogeneous, interminable, level, nonstop, perpetual, continual, recurrent, sequential, unchanging, unending, unchangeable, undying, even, endless, everlasting, coherent, gapless, incessant, unremitting, repetitive, invariable, lasting, perennial, cyclical, repeated, uniform, habitual, consecutive, un-intermittent, robot-like, continuing, straight, ceaseless, featureless, undeviating, connected.
never-ending (adjective)
ceaseless.
persevering (adjective)
unflagging, enduring, stoical, surviving, persevering, lasting, unwavering, tenacious, unfaltering, brave, abiding, steadfast, indomitable, patient, persistent.
resolute (adjective)
persistent, tenacious, steely, dauntless, plucky, valiant, unchangeable, unwavering, thorough, determined, strong-minded, relentless, committed, spunky, hearty, headstrong, loyal, firm, dedicated, uncompromising, strong-willed, staunch, serious, unyielding, implacable, steady, constant, dogged, insistent, devoted, willing, true, inexorable, steadfast, faithful, bold, persevering, intent, resolved, indomitable, ruthless, fearless, decided, tireless, unflinching, diligent, earnest, unshakeable, deliberate, tough, iron-willed, courageous, reliable, stern, stubborn, set, decisive, resolute, bullheaded, zealous, unbending.
tenacious (adjective)
tough, tenacious, dogged, diligent, strong-willed, stubborn, bold, obstinate, spunky, faithful, stiff, brave, tireless, decisive, bulldogged, resolute, flintlike, unflagging, plucky, enduring, hearty, fervent, true, undaunted, strong, persevering, devoted, staunch, steadfast, bullheaded, dauntless, headstrong, stony, determined, courageous, obdurate, hard, indomitable, insistent, persistent.
untiring (adjective)
patient, relentless, inexhaustible, unflagging, persistent, persevering, determined, tireless, unfaltering, steadfast, energetic, unwearied, unremitting, diligent, vigorous, unwavering, dogged, unwearying, unflinching, active.

n.

vital, alive, lively, vigorous, animated, dynamic. indefatigable (noun)
tireless, energetic, unwearying, unflagging.
inexhaustible (noun)
unwearied, active.

Other synonyms:

Weariless, unwearied. tireless
inexhaustible.
Other relevant words:
Weariless, inexhaustible, animated, vital, unwearied, active, lively, unwearying, dynamic, vigorous, energetic.

Usage examples for indefatigable

  1. He was indefatigable in his cause. – The Strength of Gideon and Other Stories Strength of Gideon; Mammy Peggy's Pride; Viney's Free Papers; The Fruitful Sleeping of The Rev. Elisha Edwards; The Ingrate; The Case of 'Ca'line'; The Finish of Patsy Barnes; One Man's Fortunes; Jim's Probation; U by Paul Laurence Dunbar
  2. But Braddell, less grieved by the loss than shocked by the scandal, was indefatigable in his researches,- he discovered her retreat. – Lucretia, Volume 6. by Edward Bulwer-Lytton
  3. Daniel, Drayton, and others were, it is well known, indefatigable revisers of their poems; they " added and altered many times," mostly for the better, occasionally for the worse. – Lucasta by Richard Lovelace