Roku Inc., which makes streaming-media devices and software, is preparing an initial public offering it expects to launch before year-end, according to people familiar with the company’s plans.

The Los Gatos, Calif., company recently hired Morgan Stanley, Citigroup Inc. and Allen & Co. as underwriters on the IPO, these people said. The company could file confidentially in the next few weeks, one of the people said.

Roku...