Phosphorus (P) is an essential element for sustainable development of terrestrial ecosystems. Mountains cover approximately 20% of the land area. The biogeochemical cycle of P in the mountain ecosystems differs from that in lowland ecosystems because of the huge spatial heterogeneity, fast transport of materials, and relatively rich biodiversity. As global changes, the increase in temperature, atmospheric CO₂ concentration, and N deposition have disrupted the stoichiometric equilibrium of C, N, and P. In mountain ecosystems, especially alpine regions that are sensitive to global changes, P bioavailability has become a hot area in the field of biogeochemical-cycle research. Nearly three times as many articles on the biogeochemical cycle of P in mountain ecosystems were published in the last decade in comparison with the century before. This review summarizes the characteristics of P bioavailability in mountain ecosystems; its effects on plant biodiversity and primary productivity are comprehensively explored. The P bioavailability in mountain ecosystems differs from that in lowland ecosystems in three ways: (1) there is obvious spatial heterogeneity in P bioavailability (because of steep elevation and topographic gradients), which exists on different spatial scales and in various forms depending on the factors influencing the biogeochemical cycle of P; (2) the P bioavailability in the mountains is significantly affected by the fast P transport, because hydrologic and gravitational processes are accelerated by the steep elevation gradient; and (3) biological processes are the key factors influencing the P bioavailability in the mountains. Plants and microorganisms act both as active pools and regulators in the P cycle. It is generally accepted that diversity of plant species reaches its maximum at moderate P bioavailability. Under conditions of low and high P bioavailability, P limitations and competitive exclusion may suppress the diversity of plant species. Nevertheless, in mountain ecosystems with complex structures, some mechanisms (including disturbance, niche separation, and rich biodiversity) can alleviate this suppression; thus, plant species diversity may be affected to a lesser degree by the changes in P bioavailability. In terms of the primary productivity, limitations on P bioavailability can be found in mountain ecosystems worldwide according to field surveys and nutrient manipulation experiments. In contrast to the croplands featured by single species, the responses of primary productivity in the mountain ecosystems to P bioavailability can be affected by composition of the set of plant species. When P bioavailability decreases, species with high P utilization efficiency can become more dominant; thus, the primary production may not decrease substantially. In addition, environmental factors such as temperature, water, wind, and radiation in mountain ecosystems may further weaken the responses of primary productivity to P bioavailability. Therefore, just as the plant species diversity, the primary productivity in mountain ecosystems with complex structure may be less sensitive to the changes in P bioavailability. The P bioavailability in the mountains is more sensitive to global changes (e.g., an increase in temperature and N deposition) because these regions generally have low temperature and N limitations. An increase in temperature and N deposition can alter the P bioavailability in mountain ecosystems directly or indirectly. Meanwhile, P bioavailability can, to a certain degree, affect the responses of mountain ecosystems to global changes. Long-term field observation and nutrient manipulation experiments, together with the use of new P analysis techniques such as X-ray absorption near-edge structure (XANES) spectroscopy and nuclear magnetic resonance (NMR) open up new opportunities to elucidate the P status, its changing trends, and its influences on mountain ecosystems. On the basis of this knowledge, it is possible to achieve better adaptation to the changing nutrient status in mountain ecosystems.