Adjectives that begin with the letter U are listed in this post.
ultimate, ultra, ultra-orthodox, ultrasonic
unabashed, unabated, unable, unabomber, unacceptable, unaccompanied, unaccounted, unaccustomed, unaddressed, unadjusted, unadorned, unaffected, unaffordable, unalienable, unalluring, unalterable, unaltered, unambiguous, unanimous, unannounced, unanswerable, unanswered, unanticipated, unapologetic, unappealing, unapproved, unarmed, unassailable, unassisted, unassuming, unattended, unattractive, unaudited, unauthorized, unavailable, unavoidable, unaware, unbalanced, unbearable, unbelievable, unbending, unbiased, unblinking, unborn, unbridled, unbroken, uncalled, uncanny, uncertain, unchallenged, unchanged, unchanging, uncharacteristic, uncharted, unchecked, unclaimed, unclean, unclear, unclothed, uncomfortable, uncommon, uncompetitive, uncomplicated, uncompromising, unconcerned, unconditional, unconfirmed, unconnected, unconscious, unconstitutional, uncontested, uncontrollable, uncontrolled, unconventional, unconvinced, unconvincing, uncooperative, uncorrected, uncouth, uncritical, uncut, undamaged, undated, undaunted, undecided, undefeated, undefined, undemocratic, undeniable, underage, underappreciated, undercover, underdeveloped, underemployed, undergraduate, underground, underpaid, under-performing, underprivileged, underrated, under-reported, underserved, undersized, understandable, under-the-table, underwater, underworked, undesirable, undetected, undetermined, undeterred, undeveloped, undisciplined, undisclosed, undisputed, undistinguished, undisturbed, undreamed, undue, uneasy, uneducated, unelected, unemployed, unencumbered, unending, unenforceable, unenthusiastic, unequal, unequivocal, unethical, uneven, uneventful, unexpected, unexplained, unexplored, unfair, unfamiliar, unfashionable, unfathomable, unfavorable, unfazed, unfeasible, unfertilized, unfettered, unfinished, unfit, unflappable, unflattering, unflinching, unforeseen, unforgettable, unforgiving, unfortunate, unfounded, unfree, unfriendly, unfulfilled, unfunded, ungainly, unglamorous, unglued, ungovernable, unhappy, unharmed, unhealthy, unheard, unheard-of, unheralded, unheroic, unhindered, unholy, unhurt, unidentified, unified, uniform, uniformed, unilateral, unimaginable, unimpeachable, unimpeded, unimportant, unimpressed, uninfected, uninformative, uninformed, uninhabitable, uninhabited, uninhibited, uninitiated, uninspected, uninspired, uninsured, unintelligible, unintended, uninterested, uninterrupted, unionized, unique, unissued, unitarian, unitary, united, universal, university-based, unjust, unjustified, unkempt, unkind, unknowable, unknown, unlawful, unleaded, unlicensed, unlikely, unlimited, unlisted, unlucky, unmanageable, unmanned, unmarked, unmarried, unmatched, unmet, unmistakable, unmistakeable, unnamed, unnecessary, unneeded, unnerved, unnoticed, unoccupied, unofficial, unopened, unopposed, unoriginal, unorthodox, unpacked, unpaid, unpalatable, unparalleled, unperturbed, unplanned, unpleasant, unpopular, unprecedented, unpredictable, unprepared, unpretentious, unproductive, unprofessional, unprofitable, unprotected, unproven, unpublished, unpunished, unqualified, unquestionable, unrealistic, unrealized, unreasonable, unrecognized, unrecoverable, unregistered, unregulated, unrelated, unreleasable, unreleased, unrelenting, unreliable, unremarkable, unrepentant, unreported, unresolved, unresponsive, unrestrained, unrestricted, unrivaled, unruly, unsafe, unsatisfactory, unsatisfying, unsavory, unscathed, unscheduled, unscientific, unscripted, unscrupulous, unseated, unsecured, unseemly, unseen, unsentimental, unserious, unsettled, unsettling, unshakable, unsightly, unsigned, unskilled, unsold, unsolicited, unsolved, unsound, unspeakable, unspecified, unspent, unspoiled, unstable, unstinting, unstoppable, unstructured, unsubstantiated, unsuccessful, unsuitable, unsupported, unsure, unsuspecting, unsustainable, unsympathetic, untamed, untapped, untarnished, untenable, untested, unthinkable, untimely, untold, untouchable, untouched, untrained, untreated, untried, untrue, untrustworthy, unturned, unusable, unused, unusual, unviable, unwanted, unwarranted, unwary, unwashed, unwed, unwelcome, unwholesome, unwieldy, unwilling, unwise, unwitting, unworkable, unworthy, unwritten, unyielding
up-and-coming, upbeat, upcoming, upended, up-front, uphill, upmarket, up-market, up-or-down, upper, upper-class, upper-income, upper-middle, upscale, upset, upstart, upstate, uptempo, uptight, up-tight, up-to-date, upward
urban, urbane, urbanites, urgent
usable, useful, useless, usual
utilitarian, utopian, utter