Waterproof and well insulated ski wear is needed for skiing/snowboarding in Niseko. While other ski resorts and areas in Japan are colder, temperatures during December to mid-March will generally be in the negatives to -10 range. Sometimes it can reach as cold as -20 degrees for night skiing near the top of the mountain.
Thermal underwear should also be worn by beginners and those that feel the cold. When not skiing, warm waterproof boots with grip (ice/snow is very slippery to walk on) are highly recommended. Slip-on spikes can be purchased in Niseko and recommended for walking around town but be sure not to wear any spikes inside your accommodation as they will damage the floors.