The cell refractive index has been proposed as a putative cancer biomarker of great potential, being correlated with cell content and morphology, cell division rate and membrane permeability. We used digital holographic microscopy to compare the refractive index and dry mass density of two B16 murine melanoma sublines of different metastatic potential. Using statistical methods, the distribution of phase shifts within the reconstructed quantitative phase images was analyzed by the method of bimodality coefficients. The observed correlation of refractive index, dry mass density and bimodality profile with the metastatic potential of the cells was validated by real time impedance‐based assay and clonogenic tests. We suggest that the refractive index and bimodality analysis of quantitative phase image histograms could be developed as optical biomarkers useful in label‐free detection and quantitative evaluation of cell metastatic potential. Quantitative phase images (QPIs) of digital holographic microscopy were acquired for two sublines of B16 murine melanoma cells having different metastatic potential. The cellular optical and digital signature (as refractive index, dry mass density and bimodality profile of QPIs' histograms) of these cells correlates to their metastatic potential, as proved by clonogenic and proliferation tests, and constitutes a promising label‐free biomarker of malignancy.