联氨、联胺或肼（hydrazine）（分子式：N₂H₄、H₄N₂或H₂N-NH₂），是无色的剧毒化合物。致死量为小鼠口服LD50为59mg/kg，静脉注射LD50为57mg/kg。其一水合物N₂H₄·H₂O称作水合联氨或水合肼。

常态下呈无色油状液体。气味类似氨，溶于水、醇、氨等溶剂，常用于人造卫星及火箭的燃料、锅炉的抗腐蚀剂、炸药与抗氧化剂等。

联氨有吸湿性，在空气中发烟。燃烧会呈紫色火焰。液体中分子以二聚体存在。有强还原性和腐蚀性，能侵蚀玻璃、橡胶、皮革、软木等。

Hydrazine is an inorganic compound with the chemical formula N
2H
4 (also written H
2NNH
2). It is a colorless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. As of 2000, approximately 120,000 tons of hydrazine hydrate (corresponding to a 64% solution of hydrazine in water by weight) were manufactured worldwide per year. Hydrazine is mainly used as a foaming agent in preparing polymer foams, but significant applications also include its uses as a precursor to polymerization catalysts and pharmaceuticals. Additionally, hydrazine is used in various rocket fuels and to prepare the gas precursors used in air bags. Hydrazine is used within both nuclear and conventional electrical power plant steam cycles as an oxygen scavenger to control concentrations of dissolved oxygen in an effort to reduce corrosion.