For nearly 70 years, the addition of boron-X sigma bonds to carbon-carbon multiple bonds has been employed in the preparation of organoboron reagents. However, the significantly higher strength of boron-oxygen bonds has thus far precluded their activation for addition, preventing a direct route to access a potentially valuable class of oxygen-containing organoboron reagents for divergent synthesis. We herein report the realization of an alkoxyboration reaction, the addition of boron-oxygen sigma bonds to alkynes. Functionalized O-heterocyclic boronic acid derivatives are produced using this transformation, which is mild and exhibits broad functional group compatibility. Our results demonstrate activation of this boron-O sigma bond using a gold catalysis strategy that is fundamentally different from that used previously for other boron addition reactions.