Claisen may refer to:
Rainer Ludwig Claisen, a German chemist Claisen rearrangement, a reaction of a allyl vinyl ether to a γ,δ-unsaturated carbonyl Claisen condensation, a reaction between esters and carbonyl compounds in the presence of a strong base Ireland–Claisen rearrangement, a chemical reaction of an allylic ester with strong base
See also 5243 Clasien, a minor planet