In organic chemistry, 3-hydroxybutanal (acetaldol, aldol) is an organic compound with the formula CH₃CH(OH)CH₂CHO and the structure H₃C−CH(OH)−CH₂−CH=O. It is classified as an aldol (R−CH(OH)−CHR'−C(=O)−R") and the word "aldol" can refer specifically to 3-hydroxybutanal. It is formally the product of the dimerization of acetaldehyde (CH₃CHO). A thick colorless or pale-yellow liquid, it is a versatile and valuable intermediate with diverse impacts. The compound is chiral although this aspect is not often exploited.

Acetaldehyde dimerizes upon treatment with aqueous sodium hydroxide:

This is the prototypical aldol reaction.

Dehydration of 3-hydroxybutanal gives crotonaldehyde. Distillation of 3-hydroxybutanal is sufficiently forcing to effect this conversion:

Hydrogenation of 3-hydroxybutanal gives 1,3-butanediol:

This diol is a precursor to 1,3-butadiene, precursor to diverse polymers.

Polymerization of 3-hydroxybutanal is also spontaneous, but can be stopped with the addition of water.

Aldol has been used in making perfumes and in ore flotation.