Longitudinal and interventional pain studies require reliable outcome measures. Self-reported pain intensity and phasic heat pain (PHP) unpleasantness are widely used gold-standard measures. Secondary hyperalgesia area elicited by PHP is increasingly used as an outcome measure in experimental pain studies. However, the test-retest reliability and stability of these measures across multiple sessions remain unclear. Twenty-four healthy participants (12 males, 12 females) attended three sessions, each including a familiarization procedure and a PHP calibrated to 50/100 pain intensity. Pain intensity ratings, pain unpleasantness ratings, and the area of secondary hyperalgesia were recorded after PHP. Intraclass correlation coefficients (ICC) indicated good reliability for pain intensity (ICC = 0.755) and secondary hyperalgesia (ICC = 0.840), and moderate reliability for pain unpleasantness (ICC = 0.636). Repeated-measures ANOVA revealed low stability for self-reported pain intensity and unpleasantness across sessions (ps < 0.005), whereas secondary hyperalgesia showed high stability (p = 0.296) supported by Bayesian analysis (BF01 = 3.36). Validation in an independent dataset confirmed moderate reliability for PHP pain intensity (ICC = 0.718). These findings suggest that while all PHP outcome measures show moderate-to-good reliability, only secondary hyperalgesia area remains stable across sessions, highlighting its potential as a robust measure in longitudinal and interventional pain research.