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Postweaned Iberian piglets (10.5 ± 0.1 kg BW) were randomly assigned to 5 experimental isoenergetic diets (14.2 MJ ME/kg DM) based on corn, barley, and soybean meal (10 piglets/diet). Diets provided increasing Lys concentrations (7 to 11 g Lys/kg DM) at a constant CP content (166 ± 0.5 g/kg DM); the rest of the nutrients remained constant and in sufficient amounts. Piglets were fed ad libitum until 25.1 ± 0.4 kg BW, when 5 piglets per treatment were slaughtered. Feed intake and BW were recorded daily and weekly, respectively. At slaughter, viscera were weighed and samples of small intestine from the duodenum, jejunum, and ileum quickly were collected for microanatomical analyses to test the effect of Lys concentration of dietary protein on intestinal morphology. Microtome sections of 3 to 5 μm of thickness were performed and stained with hematoxylin and eosin. Villus height, villus diameter, crypt depth, and crypt diameter were determined. Average daily feed intake was not modified by dietary Lys concentration. Average daily gain and G:F linearly increased (P< 0.001) on increasing Lys content. The effect of dietary Lys on gut morphology differed between intestinal segments. In the duodenum, villus height decreased on increasing Lys concentration following linear and cubic patterns (P= 0.002); the villus height:crypt depth ratio (V:C) followed the same pattern as villus height. In the jejunum and ileum, a linear and cubic increase on villus height was observed as dietary Lys increased (P< 0.02–0.001). In the jejunum, the V:C was not affected on increasing dietary Lys, whereas in the ileum, the V:C followed an increasing cubic pattern (P= 0.002). Crypt depth was, in general, not affected. Although increased villus height in the jejunum and ileum could contribute to the enhanced performance observed as dietary Lys increased, further studies are required to investigate the relevance of physiological effects of dietary Lys on gut function and its involvement on changes in intestinal morphology of piglets.