The continental crust is produced by the solidification of aluminosilicate‐rich magmas which are sourced from deep below the surface. Migration of the magma depends on the density (ρ) contrast to source rocks and the melt viscosity (η). At the surface, these silica‐rich melts are typically sluggish due to high η > 1,000 Pa s. Yet at their source regions, the melt properties are complexly influenced by pressure (P), temperature (T), and water contents (XH2O ${X}_{{\mathrm{H}}_{2}\mathrm{O}}$). In this study, we examined the combined P‐T‐XH2O ${X}_{{\mathrm{H}}_{2}\mathrm{O}}$ effects on the behavior of melts with an albite stoichiometry (NaAlSi3O8). We used first‐principles molecular dynamics simulations to examine anhydrous (0 wt % H2O) and hydrous (5 wt % H2O) melts. To constrain the P and T effects, we explored P ≤ 25 GPa across several isotherms between 2500 and 4000 K. The melts show anomalous P‐ρ relationships at low P ∼ 0 GPa and high T ≥ 2500 K, consistent with vaporization. At lithospheric conditions, melt ρ increases with compression and is well described by a finite‐strain formalism. Water lowers the melt density (ρhydrous < ρanhydrous) but increases the compressibility, that is, 1/Khydrous >1/Kanhydrous or Khydrous < Kanhydrous. We also find that the melt η decreases with pressure and then increases with further compression. Water decreases the viscosity (ηhydrous < ηanhydrous) by depolymerizing the melt structure. The ionic self‐diffusivities are increased by the presence of water. The decreased ρ and η by H2O increase the mobility of magma at crustal conditions, which could explain the rapid eruption and migration timescales for rhyolitic magmas as observed in the Chaitén volcano in Chile. Plain Language Summary The continental crust is produced by solidifying aluminosilicate‐rich magmas. Such magmas are known to be highly viscous at the surface. It is expected that the magmas will become more viscous due to increasing pressure at the deep crustal depths near their sources. However, observations contrast the expectations. Some volcanic eruptions indicate rapid movement of the aluminosilicate‐rich magmas before the eruption. The movement of magma is influenced by its density and viscosity. These properties are influenced by pressure, temperature, and the water contents of the magma. To better understand how these parameters affect magmas in the crust, we performed computer simulations on molten albite with and without water. The albite chemistry mimics the chemistry of aluminosilicate‐rich magmas in the crust. At conditions of the deep crust where magmas originate, the densities of the magmas increase with compression. The magma viscosities also decrease under the same pressure. Our results provide, in part, a plausible explanation for a surprisingly rapid eruption of the Chaitén volcano in Chile. Water lowers both the magma density and viscosity which helps to explain the rapid eruption of the hydrous aluminosilicate‐rich lavas. Key Points We constrain the combined effects of pressure, temperature, and water contents on aluminosilicate melts with an albitic composition In the crust, relatively low temperatures should increase melt viscosity. Yet pressure and H2O lower the viscosity at these conditions Our results provide a plausible explanation for rapid eruption of lavas which are typically very viscous and should hence be sluggish