Is it a unique or an unique?

The correct article is "a unique." For example:

• a unique idea
• a unique individual
• a unique house

Using a or an depends on the sound (consonant or vowel) at the beginning of a Word, not the letter, so when a "u" (vowel) Word is pronounced as though it begins with a "y" (consonant: yoo nique), it's treated more like the consonant sound of the y.