Harley-Davidson Inc. plans to close one of its four U.S. factories amid a protracted sales slide that the motorcycle maker expects to continue into 2018.

The Milwaukee-based company said Tuesday that motorcycle-related revenue fell 6.8% in 2017 to $4.92 billion. Including a decline in the U.S., hog retail sales fell 7% world-wide in 2017, the third consecutive annual decline.

As...