The Waikato Plains are a large area of low-lying land in the northwest of the North Island of New Zealand. The plains are the floodplains of the country's longest river, the Waikato.

The plains can be divided roughly into the Middle Waikato Plain, extending in all directions around the city of Hamilton, and the Lower Waikato Plain, to the east of the river's mouth. The two are broken by the rough, low-lying hills of the Hakarimata Range between Ngaruawahia and Huntly.

The region is heavily populated by New Zealand standards, with over 250,000 people in approximately 8000 km² area. Just under half of these live in Hamilton.

The plains are an area of very fertile land, which is intensively farmed. Dairy cattle, sheep, grain and maize are all farmed here, but it is dairy that is the staple of the local economy. The lower plain is also known for vineyards, and the middle plain has some of the southern hemisphere's top thoroughbred stables.

A considerable amount of the land is peaty, and some sections are undrained swamp. Several shallow riverine lakes lie at the southern end of the lower plain, notably Lake Waikare.