Sipiwesk Lake is a large lake in northern Manitoba, Canada, situated at approximately 55°05′N 97°34′W within Division No. 22. It has a surface area of 454 km² (175 sq mi) and a maximum length of 78 km (48 mi), with a surface elevation of 183 m (600 ft). The lake forms an expansion of the Nelson River, which enters the south end of Sipiwesk on each side of Bear Island, flowing through Duck Lake to the west of Bear Island before exiting from the lake's east end. This elongated form and its position within the Nelson River watershed give it a distinctly riverine character despite its considerable size.

Access is limited given the lake's remote northern location. The Sipiwesk Lake Access Road provides seasonal road connectivity, though spring weight restrictions typically take effect in late March or early April to protect thawing ground. Boat access is available from Sipiwesk Landing, a gauging station site on the lake. Floatplane charters from nearby Thompson are another common access option for this part of northern Manitoba.

Paddlers should be aware that the lake is affected by upstream hydroelectric infrastructure, including the Jenpeg Dam. Documented accounts describe the lake experiencing significant flooding, with water levels running as much as nine to ten feet above normal, resulting in submerged trees along the shoreline, rapidly eroding banks, extensive driftwood, and inaccuracies between on-the-water conditions and topographic maps. Islands and points may be partially or fully submerged, and peninsulas can appear as islands under high-water conditions. Camping can be difficult under such circumstances due to drowned shoreline vegetation.

At the north end of the lake, where the Nelson River exits, a portage trail connects to Landing Lake via Sabomin Lake. This connection, historically known as Cross Portage, was the site of a Hudson's Bay Company outpost from approximately 1885 to 1889. The Nelson River section downstream of Sipiwesk is noted for strong current, which historically led voyageurs to favor other routes such as the Hayes River.

Fishing regulations place Sipiwesk Lake within Manitoba's North Central Division. The Nelson River and its tributaries from Whitemud Falls to Kelsey Generating Station, including Sipiwesk Lake, are closed to lake sturgeon fishing from May 1 through June 15. The surrounding boreal forest supports wildlife including loons and eagles. The lake is described by paddlers as scenic, with the section of the Nelson River at and upriver of Sipiwesk considered among the more visually appealing stretches of that river system.