Te Waihora / Lake Ellesmere is a shallow, brackish coastal lagoon in the Canterbury region of New Zealand's South Island, lying directly west of Banks Peninsula and separated from the Pacific Ocean by the long, narrow Kaitorete Spit. Covering approximately 20,000 hectares — comparable in area to Lake Wānaka — it is Canterbury's largest lake and New Zealand's fifth largest by area. Despite its vast extent, the water is exceptionally shallow, averaging just 1.4 metres in depth with a maximum of around 2.7 metres near Kaituna. The lake is technically a coastal lagoon, periodically opened to the sea by cutting a channel through the southern end of Kaitorete Spit, which affects salinity and water levels throughout the year.

Kayaks can be launched from boat ramps at Wolfes Road on the Ararira/LII River, at Lower Selwyn Huts on the Waikirikiri/Selwyn River, and at Lakeside Domain on the western end of the lake. Lakeside Domain also offers camping facilities directly on the lake's edge and is a hub for water sports including jet skiing, windsurfing, sailing, and kite boarding. The eastern shore near Kaituna is accessible via the Little River Rail Trail, which runs alongside the lake margin and connects Christchurch to Little River.

Several significant safety hazards require attention. Toxic cyanobacteria (algal) blooms occur on the lake and are described as extremely hazardous to both humans and animals, including dogs. Paddlers are strongly advised to check the current lake status with Environment Canterbury before visiting. Weather changes can cause serious fluctuations in water levels, and care is needed if entering the water. The native stinging swamp nettle is also present around the lake margins.

The lake holds exceptional ecological value, with over 200 bird species recorded — making it the most diverse birdlife site in New Zealand. Notable species include migratory waders such as the wrybill, curlew sandpiper, and red-necked stint, as well as black swan, paradise shelduck, and pukeko. The Greenpark Sands area is particularly good for spotting migratory species, and a bird hide at Waitatari/Harts Creek offers views over the lake. Water quality is a known concern, as the lake is classified as eutrophic and receives nutrient-laden runoff from its 276,000-hectare catchment. The water is described as murky and shallow throughout. The lake carries deep cultural significance to Ngāi Tahu, for whom it is a major mahinga kai site.