Ice-binding proteins (IBPs), produced by polar and cold-tolerant organisms, have the ability to bind to ice, affecting its growth. They are key elements in biological adaptation to cold environments, and no other particles, either natural nor synthetic, show a comparable effect in controlling ice growth. Here we present the IBPs from the polar sea-ice diatom Fragilariopsis cylindrus (fcIBP). This protein can shape the ice and allows survival of the diatom within sea-ice. It belongs to a protein family defined by a domain (DUF 3494) that is extremely successful among marine polar microorganisms but is also identified in other habitats such as, for example, an Antarctic ice-core. In order to shed light on the details of the interaction between IBPs and ice, as a first step leading to a better understanding of the effect of the proteins in their natural icy environment, we studied the effects of fcIBPs on single crystal free growth. Different IBP families affect ice in different ways and the relevant common traits, as well as the differences of the ice binding mechanisms, are still under investigation. We analyzed crystal morphological changes and crystal growth rates dependent on supercooling and fcIBP concentration, applying optical bright field and interferometric microscopy. We saw differential effects of the protein on the growth of the different crystallographic planes, revealing a new pattern of IBP–ice interaction. Furthermore, switching to a more macroscopic level, we analyzed the effect of fcIBPs on the physical properties of polycrystalline ice. We observed the evolution of microstructure in fine-grained ice samples over longer time periods (several weeks), and were able to show a strong inhibition by fcIBPs of grain growth. Also, we showed that the effect of IBPs on the driving factors for ice deformation during creep, i.e. on internal dislocations due to incorporation within the lattice and on the mobility of grain boundaries due to pinning, make these proteins particularly interesting in studying the process of ice deformation. Our results of ice single crystal growth and of the microstructure evolution of polycrystalline ice in the presence of fcIBPs show that these proteins have remarkable properties that make them suited to basic understanding of the mechanisms of grain growth, recrystallization and deformation processes, but also to be adapted for industrial applications wherever ice grain control is of interest.