WASHINGTON—The U.S. government ran a $100 billion deficit last month, compared with a $63 billion budget gap in October 2017, as spending growth outpaced revenue growth, the U.S. Treasury said Tuesday.

The Treasury Department said federal outlays rose 18% in the first month of the 2019 fiscal year compared with the same period a year earlier, in large part because of higher spending on Medicare, national defense and interest payments on the debt.