Functional materials are frequently used to identify and characterize proteins and their modifications in complex biological samples throughout the analytical workflow. Protein phosphorylation is a highly important post-translational modification that is being examined with the help of diverse materials that exhibit affinity towards the phosphate group of phosphoamino acids. Titanium dioxide and zirconium dioxide particles are increasingly used to enrich phosphopeptides from proteolytic digests of protein mixtures, although specificity and recovery still leave room for improvement. Here, we present tin dioxide (SnO2, stannia) microspheres as a new type of metal oxide material for phosphopeptide enrichment. The microspheres are produced by a nanocasting process, starting from silica particles as a template, which allows the tuning of material properties such as particle diameter and porosity of the microspheres in a straightforward manner. For the first time, we are able to show that tin dioxide can be used to enrich phosphopeptides from mixtures such as enzymatic digests of proteins, followed by analysis by liquid chromatography-mass spectrometry (LC-MS). For optimization of the enrichment protocol, we use synthetic phosphorylated and nonphosphorylated peptides, and test different solvent compositions for loading and washing steps to enhance the selectivity of the material without compromising phosphopeptide recovery. Furthermore, the Selectivity and phosphopeptide binding properties of tin dioxide are compared to the established metal oxide materials, titanium dioxide and zirconium dioxide, using mixtures of model proteins. Even without the use of additional additives such as alpha-hydroxy acids, which have been used to enhance the specificity of TiO2-based enrichment, we show that a comparably good performance can be achieved for the SnO2 spheres.