Google Inc. said Monday that it will reduce Motorola Mobility's workforce by about 20% to help streamline the unprofitable wireless-phone maker.

The roughly 4,000 job cuts are the first large-scale layoffs in Google's 13-year history. They are aimed at returning Motorola's once-dominant mobile-devices unit to profitability after the business spent 14 of the past 16 quarters in the red, and come as analysts speculate about whether Google is considering a spinoff of its home-television business that provides set-top boxes and...