The direct growth of planar-twinned Au nanoparticles (NPs) in high yield remains a challenge in shape-controlled NP synthesis largely because suitable planar-twinned seeds for Au NP growth have not been identified to date. Herein we describe the use of planar. twinned Ag triangular nanoprisms as a means to dictate Au NP twin structure. In a one-pot process, the Ag triangular nanoprisms first undergo oxidative Au replacement, forming Ag-Au alloy nanoframes and concomitantly releasing Ag+ into solution, which then directs subsequent Au NP growth through an underpotential deposition process. The planar-twinned structure of the initial Ag nanoprism is maintained throughout particle growth. Using this method, we have successfully synthesized Au hexagonal bipyramids in high yield for the first time.