In September you’ll find all the park roads, services, and campgrounds are open so it’s one of the best times to visit Yellowstone National Park.
The average temperatures in September at Yellowstone are highs in the upper 60s and lows in the upper 30s degrees Fahrenheit.
We suggest you plan to go for at least 4-5 days to really experience Yellowstone. This way you are not rushed and you can be a little more patient with the crowds.
Before exploring any Yellowstone hiking trails in September, we recommend you check with the rangers at the Visitor Center to get current trail conditions.