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.

Some of the deterioration in October can be explained by quirks in the calendar. Oct. 1 last year fell on a Sunday, so government payments due that day were made the previous Friday, shifting some outlays to September. That held down October 2017 payments and made October 2018 payments worse by comparison.

If not for the timing shifts, last month’s deficit would have been $1 billion less than the same period last year, according to Tuesday’s budget statement.

Government receipts were up 7% in the month of October compared with October 2017, though looked at over a longer stretch they are lagging. Added up over 12 months, federal revenues were up 0.5% from the previous 12-month period, lagging the pace of broader economic growth.