January: Winter turns Yellowstone into a different park. Bison wear frost, geyser steam towers in the cold air, and Lamar Valley is one of the most reliable places on Earth to watch wild wolves.
June: Bison calves ('red dogs') and elk calves are everywhere, grizzlies are active in the valleys, and the geysers steam under long early-summer days. Crowds are lighter than in July and August.
September: September is one of the best months: summer crowds drop sharply after Labor Day, elk bugle at Mammoth, and bison and bears are active in cool weather.
Permits and reservations
No vehicle reservation is needed to enter. An entrance pass is required, and you can buy it ahead online or at any entrance.
Boating permit: Non-motorized boats are $20 for 7 days or $30 for the season, and motorized boats are $40 or $60. This includes the required aquatic invasive species inspection.
Campgrounds: Reservations open on a rolling basis exactly 6 months before your stay. Most sites book far ahead.
Backcountry early-access lottery: Apply March 1–20 ($10). Winners get a random access time between April 1 and 24. Online reservations must be made at least 3 days before the trip.