Due to the excellent strength/weight ratio, good energy absorbing ability, superior thermal, and acoustic insulation properties together with reasonable costs, polymer foams are the material of choice for many applications. The incorporation of particulate or fibrous fillers can even improve the properties. In this article, we investigate an open-cell polyurethane foam filled with cellulose particles. This is a new moisture-absorbing wood-plastic composite used for special mattresses. The foam was characterized by high-resolution cone-beam X-ray computed tomography (XCT) with voxel sizes between 0.7 μm3 and 2.2 μm3. The XCT measurements were performed with a tube voltage of 50 kV to obtain enough contrast. The XCT data were processed by various algorithmic steps (e.g., smoothing, thresholding, watershed transformation, erosion, dilation, and feature extraction) to ascertain the three-dimensional open-cell structure together with particle distribution, size, and position. Quantitative data for the cells (mean diameter, shape, volume, and position), for the cellulose particles (mean diameter, shape, volume, and position), for the polymer vertices (shape and position) and for combinations of them (e.g., distance between particles and vertices) were derived from the XCT data. 15.2 cells/mm3, 41 cellulose particles/mm3, and 88.7 polymer vertices/mm3 were found. The average diameter value for the mostly globular cells was found to be 465 μm with a Gaussian-like distribution function. The cellulose particles are globular and elongated and the distribution resembles an exponential function where 95% of the particles have diameters below 60 μm. The particles are situated within the polymer walls and close to the surface. The function for the distance of the particles from the closest vertices corresponds to a Weibull distribution function with a scale parameter of 53 μm and a shape parameter of 1.7.