Due to its unique capability of visualizing optical absorption in deep tissues, photoacoustic tomography is increasingly used in biomedical imaging. Among various types of transducer arrays, the linear array is perhaps the most widely used in photoacoustic tomography because it is commercially available and readily allows ultrasound imaging. However, the three-dimensional imaging capability of a linear array is limited due to its poor elevational resolution. While various scanning schemes have been proposed to address this problem, they all suffer from long scanning time to the best of our knowledge. To address this issue, we introduce slit-enabled three-dimensional photoacoustic tomography. The metal slit, placed at the array focus, causes the incoming photoacoustic waves to diffract along the elevation direction and, hence, significantly improves the elevation detection aperture and resolution. We tested the new system in both phantoms and animals. The slit improves the elevation resolution by 10 times without compromising scanning time.