The in vitro bioactivity of a composite composed by a biodegradable starch-based polymeric matrix and hydroxyapatite fillers was investigated, in situ, as a function of immersion time in a simulated body fluid (SBF) using atomic force microscopy (AFM). The surface roughness of the composite started to increase after the initial 8 h because of both the degradation of the polymer matrix and the nucleation of calcium phosphate. After 24 h of immersion the surface of the composite was fully covered with calcium phosphate nuclei with diameters around 126 nm. As the immersion time increased, the nuclei increased both in number and size, and coalesced leading to the formation of a dense and uniform calcium phosphate layer on the surface of the composite only after 126 h of SBF immersion. The results of in situ AFM observation agreed with those of standard in vitro bioactivity tests in combination with scanning electron microscopy observations. Thin-film X-ray diffraction demonstrated that the ratio of apatite to the polymer matrix was higher within the surface layer (40 μm deep from the surface) than that in the bulk after the immersion for 7 days. The water-uptake capability of the polymer contributes to the nucleation and growth of the calcium phosphate layer. These results suggest the great potential of the composite for a range of temporary applications in which bone-bonding ability is a desired property.