Disiamylborane (bis(1,2-dimethylpropyl)borane) is an organoborane with the formula [((CH3)2CHCH(CH3))2BH]2 (abbreviation: Sia2BH).

It is a colorless waxy solid that is used in organic synthesis for hydroboration–oxidation reactions.

[1] The reaction stops at the secondary borane due to steric hindrance.

The hydroboration process proceeds via an initial dissociation of the dimer.

[2] The prefix disiamyl is an abbreviation for "di-sec-isoamyl", where sec-isoamyl ("secondary isoamyl") is an archaic name for the 1,2-dimethylpropyl group (amyl being a obsolescent synonym of pentyl).