Ti-Nb-based alloys are candidate materials for load-bearing and for functional biomedical implant components. For these applications alloy compositions with relatively low martensitic transformation (MT) temperatures are of highest interest. In this work we examine the thermal stability and the temperature-induced β ↔ α″ MT of Nb-rich martensitic and partially martensitic Ti-Nb alloys exhibiting martensite start temperatures Ms < 300°C. The first part of this article investigates the phase transformations and precipitation reactions occurring during isochronal cycling in dependence of the Nb content using differential scanning calorimetry and dilatometry in combination with X-ray diffraction. The second part studies the temperatures, thermal hysteresis and transformation interval of the β ↔ α″ MT. The latent heat, elastic and irreversible energy contributions of the thermoelastic energy balance are quantified in dependence of Nb content in the remaining parts. All energy contributions decrease with increasing Nb content and the latent heat becomes very small (< 5 J/g) for the Nb-richest martensitic compositions. Above a critical Nb concentration the β → α″ MT is incomplete.