纳米颗粒（nanoparticle），指纳米量级的微观颗粒。它被定义为至少在一个维度上小于100纳米的颗粒。小于10纳米的半导体纳米颗粒，由于其电子能级量子化，又被称为量子点。

纳米颗粒具有重要的科学研究价值，它搭起了大块物质和原子、分子之间的桥梁。大块物质的物理性质通常与大小无关，但是在纳米尺寸上却通常并非如此。一些和尺寸相关的物理性质被观测到，例如：半导体纳米颗粒的量子束缚，一些金属纳米颗粒的表面胞质共振（surface plasmon resonance），磁性材料的超顺磁性。 类固体和软的纳米颗粒也被制造出来。脂质体是典型的具有类固体特性的纳米颗粒。

Nanoparticles are particles between 1 and 100 nanometers in size. In nanotechnology, a particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. Particles are further classified according to diameter. Ultrafine particles are the same as nanoparticles and between 1 and 100 nanometers in size, fine particles are sized between 100 and 2,500 nanometers, and coarse particles cover a range between 2,500 and 10,000 nanometers. Nanoparticle research is currently an area of intense scientific interest due to a wide variety of potential applications in biomedical, optical and electronic fields. The National Nanotechnology Initiative has led to generous public funding for nanoparticle research in the United States.