Animals adaptively adjust nutrient intake based on internal physiological need. Although protein deficiency elicits robust behavioral and endocrine responses, the sensory mechanisms that detect dietary protein and guide selective feeding remain incompletely understood. Here, we identify a population of vagal sensory neurons that respond selectively to intragastric protein and are required for adaptive regulation of protein intake. Using activity-dependent genetic labeling and in vivo calcium imaging, we show that these neurons are activated by dietary protein, exhibit enhanced responses in protein-restricted states, and are distinct from previously characterized calorie-sensing populations. Selective ablation of protein-responsive vagal neurons abolishes the normal shift in protein preference during protein deprivation, blunts motivation to work for protein rewards, and prevents behavioral updating following protein repletion. These neurons also mediate protein-specific satiety, limiting further protein intake without affecting carbohydrate consumption. Notably, protein preference is suppressed under mild caloric restriction, indicating that caloric and amino acid needs are hierarchically organized and likely monitored by separate interoceptive systems. Our findings reveal a novel vagal circuit that integrates internal protein status with nutrient-specific cues to guide adaptive protein appetite and maintain amino acid homeostasis.