Yes, BULL is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, L 1, L 1.

Is it a valid word?

BULL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

uncastrated adult male of domestic cattle
a large and strong and heavyset man
obscene words for unacceptable behavior
a serious and ludicrous blunder
uncomplimentary terms for a policeman
an investor with an optimistic market outlook; an investor who expects prices to rise and so buys now for resale later
(astrology) a person who is born while the sun is in Taurus
the second sign of the zodiac; the sun is in this sign from about April 20 to May 20
the center of a target
a formal proclamation issued by the pope (usually written in antiquated characters and sealed with a leaden bulla)
mature male of various mammals of which the female is called `cow'; e.g. whales or elephants or especially cattle

Also: irish bull, taurus the bull, bull's eye, papal bull, bull through, talk through one's hat.

Not playable in Scrabble US: SAMSON, DOGSHIT, TAURUS.