Nenjiang City (Chinese: 嫩江市; pinyin: Nènjiāng shì), formerly Nenjiang County, is a county-level city under the administration of Heihe prefecture-level city in northwestern Heilongjiang province, China. It is located on the river of the same name (Nen River), which also forms part of the provincial border with Inner Mongolia, more than 200 kilometres (120 mi) southwest of the urban area of Heihe. The city seat is Nenjiang Town. Land area 15,360 square kilometres (5,930 sq mi), population 500,000.

Under the name Mergen or Merghen (墨爾根, Mò'ěrgēn), Nenjiang was the provincial capital and seat of the military governor of Heilongjiang Province in 1690–1699.

Nenjiang has a monsoon-influenced humid continental climate (Köppen Dwb), with long, harsh, but dry winters, and short, very warm summers. The monthly 24-hour average temperature ranges from −23.3 °C (−9.9 °F) in January to 21.1 °C (70.0 °F) in July; the year averages out at only 0.85 °C (33.5 °F). Around 80% of the annual precipitation occurs from June to September. With monthly percent possible sunshine ranging from 53% in July to 74% in February, sunshine is generous year-round, totalling 2,726 hours annually.

Nenjiang City is divided into 9 towns and 5 townships.