A microprobe of protons with an energy of 17 million electron volts is used to quantitatively image three-dimensional hydrogen distributions at a lateral resolution better than 1 micrometer with high sensitivity. Hydrogen images of a <110>-textured undoped polycrystalline diamond film show that most of the hydrogen is located at grain boundaries. The average amount of hydrogen atoms along the grain boundaries is (8.1 ± 1.5) × 1014 per square centimeter, corresponding to about a third of a monolayer. The hydrogen content within the grain is below the experimental sensitivity of 1.4 × 1016 atoms per cubic centimeter (0.08 atomic parts per million). The data prove a low hydrogen content within chemical vapor deposition–grown diamond and the importance of hydrogen at grain boundaries, for example, with respect to electronic properties of polycrystalline diamond.