Gold nanostars are valuable materials for nanomedicine, energy conversation, and catalysis. Microfluidic synthesis offers a simple and controlled means to produce nanoparticles as they offer precise fluid control and improve heat and mass transfer. 3D-printed microfluidics are a good alternative to PDMS devices because they are affordable to produce and can be more easily integrated with active mixing strategies. 3D-printed microfluidics has only been applied to the production of silver and gold nanospheres, but not complex structures like gold nanostars. Synthesis of gold nanostars requires highly effective mixing to ensure uniform nucleation and growth. In this work, we present a 3D-printed microfluidic device that utilizes an efficient vibrating sharp-tip acoustic mixing system to produce high-quality and reproducible gold nanostars via a seedless and surfactant-free method. The vibrating sharp-tip mixing device can mix three streams of fluid across ~ 300 μm within 7 ms. The device operates with flow rates ranging from 10 μL/min to 750 μL/min at low power requirements (2–45 mW). The optical properties of the resulting nanotars are easily tuned from 650 to 800 nm by modulating the input flow rate. Thus, the presented 3D-printed microfluidic device produces high-quality gold nanostars with tunable optical and physical properties suitable for extensive applications.