Yes, KNOT is a Scrabble word!

Letter values in Scrabble US: K 5, N 1, O 1, T 1.

Is it a valid word?

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

a tight cluster of people or things
any of various fastenings formed by looping and tying a rope (or cord) upon itself or to another rope or to another object
a hard cross-grained round piece of wood in a board where a branch emerged
something twisted and tight and swollen
a unit of length used in navigation; exactly 1,852 meters; historically based on the distance spanned by one minute of arc in latitude
soft lump or unevenness in a yarn; either an imperfection or created by design
a sandpiper that breeds in the Arctic and winters in the southern hemisphere

Also: nautical mile, naut mi, international nautical mile, air mile, calidris canutus.

Not playable in Scrabble US: GREYBACK.