Background: Preterm human milk-fed infants often experience suboptimal growth despite the use of human milk fortifier (HMF). The extra protein supplied in fortifiers may be inadequate to meet dietary protein requirements for preterm infants. Objective: We assessed the effect of human milk fortified with a higher-protein HMF on growth in preterm infants. Design: This is a randomized controlled trial in 92 preterm infants born at <31 wk gestation who received maternal breast milk that was fortified with HMF containing 1.4 g protein/100 mL (higher-protein group) or 1.0 g protein/100 mL (current practice) until discharge or estimated due date, whichever came first. The HMFs used were isocaloric and differed only in the amount of protein or carbohydrate. Length, weight, and head-circumference gains were assessed over the study duration. Results: Length gains did not differ between the higher- and standard-protein groups (mean difference: 0.06 cm/wk; 95% CI: -0.01, 0.12 cm/wk; P = 0.08). Infants in the higher-protein group achieved a greater weight at study end (mean difference: 220 g; 95% CI: 23, 419 g; P = 0.03). Secondary analyses showed a significant reduction in the proportion of infants who were less than the 10th percentile for length at the study end in the higher-protein group (risk difference: 0.186; 95% CI: 0.370, 0.003; P = 0.047). Conclusions: A higher protein intake results in less growth faltering in human milk-fed preterm infants. It is possible that a higher-protein fortifier than used in this study is needed. This trial was registered with the Australian New Zealand Clinical Trials Registry (http://www.anzctr.org.au/) as ACTRN12606000525583.