Weak mechanical forces acting on molecules are in control of a wide variety of (bio)chemical and physical processes. The spatially inhomogeneous nature of these forces has a profound effect on the structure and mechanics of soft and biological materials. Yet, the lack of methods for probing sub-picoNewton forces at high resolution leaves our understanding of these effects incomplete. Here, we solve this challenge by engineering semiconducting polymers to act as ultraweak force sensors. Combining simulations, chemical synthesis, and single-molecule fluorescence spectroscopy, we demonstrate force sensing in single molecules. We achieve grayscale force detection, at a resolution as low as 300 fN, down to the molecular scale. Our approach opens the way to illuminating and quantifying molecular mechanics with unprecedented resolution. Mechanical stress at the molecular scale plays a crucial role in a wide variety of (bio)chemical processes, ranging from the sensing of the mechanical environment by cells to the failure of high-tech engineering materials. Although we know these molecular forces exist, making them visible and quantifying them at the molecular scale have remained impossible to date. This lack of direct insight at the molecular level has precluded a deeper understanding of how mechanics govern these problems. Here, we demonstrate quantitative force sensing in individual molecules at an unprecedented force resolution. Our approach is completely non-invasive and thus opens the way to visualizing and quantifying mechanical stresses in molecular materials and complex biological scenarios. Weak forces acting on molecules govern a vast range of physical, chemical, and biological phenomena. To date, it has not been possible to measure these forces directly because force-sensing methods at the nanoscale have lacked the resolution to resolve ultraweak forces at the scale of single molecules deep within complex materials. Here, we solve this challenge by demonstrating single-molecule force sensing with engineered light-emitting molecules and reporting forces as small as one trillionth of a Newton.