Strategies for the ultrafast optical control of magnetism have been a topic of intense research for several decades because of the potential impact in technologies such as magnetic memory1, spintronics2 and quantum computation, as well as the opportunities for nonlinear optical control and modulation3 in applications such as optical isolation and non-reciprocity4. Here we report experimental quantification of optically induced magnetization in plasmonic gold nanoparticles due to the inverse Faraday effect. The induced magnetic moment is large under typical ultrafast pulse excitation (