A metal-insulator-metal (MIM) capacitor structure and a method for forming the same are provided. The MIM capacitor structure includes a substrate, a bottom electrode layer, a first dielectric layer, a top electrode layer and first dielectric spacers. The bottom electrode layer is positioned over the substrate. The first dielectric layer is positioned over the bottom electrode layer. The top electrode layer is positioned over the first dielectric layer. The first dielectric spacers are positioned on opposite sidewalls of the bottom electrode layer. The first dielectric layer has a first dielectric constant. The first dielectric spacers have a second dielectric constant that is lower than the first dielectric constant.