Manufactured sands are produced by crushing rock deposits to produce a fine aggregate that is generally more angular and has a rougher surface texture than naturally weathered sand particles. Manufactured sands can also contain significant quantities of rock dust. As natural sand deposits become depleted near some areas of metropolitan growth and viable sand sources non-susceptible to alkali–silica reactivity are consumed, the use of manufactured sands as a replacement fine aggregate in concrete is receiving attention. Designers, specifiers, contractors, and material suppliers need to understand the effects of manufactured sand angularity, as well as fines content on concrete water demand and concrete durability. As part of a comprehensive research program, various manufactured sand properties and their effects on fresh and hardened concrete properties were investigated. This paper investigates the effects of manufactured sand properties on water demand of mortar and concrete. The relationships between manufactured sand characteristics and hardened concrete properties are discussed in subsequent papers. Manufactured sands with a wide range of particle angularities and fines contents were included in the testing program. The program involved measurement of various sand attributes, and a subsequent testing phase on mortar workability that isolated and evaluated the effect of the subject sand attributes on water demand and workability of mortars. The testing phase, using mortars rather than full-scale concrete mixtures, enabled the isolation of sand attributes, such as particle angularity, particle size, and fine-particle content; it also enabled the evaluation of the effect of each attribute on water demand. Results of mortar testing confirmed that particle angularity and fineness of the sand gradation (as quantified by fineness modulus) influenced the water demand of mortars. Testing performed on a paired comparison basis on individual sizes with substantially different angularities indicated that, as the particle size decreased, the exponential increase in surface area overshadowed any difference caused by particle angularity between particles of comparable size. The results of sand and mortar testing were used in the development of the subsequent testing phase, examining the effects of manufactured sand properties on concrete water demand utilizing full-scale concrete mixtures. A statistically based water demand model was developed for conventional strength concrete. The regression-based model enabled the evaluation of the contribution of each attribute and the relative importance and statistical significance of each contribution. Particle angularity was found to be the dominant contributor to water demand with the secondary contribution from fineness of the overall sand gradation, as quantified by the fineness modulus and quantity of very fine particles. The influence of these factors was statistically significant. Effects caused by well-graded particle distributions were not found to be significant, contrary to initial expectations.